Add additional NIOAsyncChannel config

Motivation:

Recent additions to pipeline configuration did not expose some of the
`NIOAsyncChannel` configuration.

Modifications:

Expose `backpressureStrategy`, `isOutboundHalfClosureEnabled`.

I think specific naming and object encapsulation for these parameters should be discussed
later before SPI is removed and we review the API as a whole.

Result:

Pipelie helpers for `NIOAsyncChannel` expose `backpressureStrategy`, `isOutboundHalfClosureEnabled`.

Sources/NIOHTTP2/HTTP2ChannelHandler+InlineStreamMultiplexer.swift

@@ -255,7 +255,11 @@ extension NIOHTTP2Handler {
         ///       or ``HTTP2Frame/FramePayload`` if there are none.
         ///     - initializer: A callback that will be invoked to allow you to configure the
         ///         `ChannelPipeline` for the newly created channel.
+        @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+        @_spi(AsyncChannel)
         public func createStreamChannel<Inbound, Outbound>(
+            backpressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.HighLowWatermark? = nil,
+            isOutboundHalfClosureEnabled: Bool = false,
             inboundType: Inbound.Type,
             outboundType: Outbound.Type,
             initializer: @escaping NIOHTTP2Handler.StreamInitializer
@@ -264,6 +268,8 @@ extension NIOHTTP2Handler {
                 initializer(channel).flatMapThrowing { _ in
                     return try NIOAsyncChannel(
                         synchronouslyWrapping: channel,
+                        backpressureStrategy: backpressureStrategy,
+                        isOutboundHalfClosureEnabled: isOutboundHalfClosureEnabled,
                         inboundType: Inbound.self,
                         outboundType: Outbound.self
                     )

Sources/NIOHTTP2/HTTP2PipelineHelpers.swift

@@ -296,6 +296,8 @@ extension Channel {
         position: ChannelPipeline.Position = .last,
         streamInboundType: StreamInbound.Type,
         streamOutboundType: StreamOutbound.Type,
+        inboundStreamBackpressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.HighLowWatermark? = nil,
+        isInboundStreamOutboundHalfClosureEnabled: Bool = false,
         inboundStreamInitializer: @escaping NIOHTTP2Handler.StreamInitializer
     ) throws -> EventLoopFuture<NIOHTTP2Handler.AsyncStreamMultiplexer<NIOAsyncChannel<StreamInbound, StreamOutbound>>> {
         if self.eventLoop.inEventLoop {
@@ -308,6 +310,8 @@ extension Channel {
                     position: position,
                     streamInboundType: streamInboundType,
                     streamOutboundType: streamOutboundType,
+                    inboundStreamBackpressureStrategy: inboundStreamBackpressureStrategy,
+                    isInboundStreamOutboundHalfClosureEnabled: isInboundStreamOutboundHalfClosureEnabled,
                     inboundStreamInitializer: inboundStreamInitializer
                 )
             }
@@ -321,6 +325,8 @@ extension Channel {
                     position: position,
                     streamInboundType: streamInboundType,
                     streamOutboundType: streamOutboundType,
+                    inboundStreamBackpressureStrategy: inboundStreamBackpressureStrategy,
+                    isInboundStreamOutboundHalfClosureEnabled: isInboundStreamOutboundHalfClosureEnabled,
                     inboundStreamInitializer: inboundStreamInitializer
                 )
             }
@@ -529,8 +535,12 @@ extension Channel {
         streamDelegate: NIOHTTP2StreamDelegate? = nil,
         connectionInboundType: ConnectionInbound.Type,
         connectionOutboundType: ConnectionOutbound.Type,
+        connectionBackpressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.HighLowWatermark? = nil,
+        connectionIsOutboundHalfClosureEnabled: Bool = false,
         streamInboundType: StreamInbound.Type,
         streamOutboundType: StreamOutbound.Type,
+        inboundStreamBackpressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.HighLowWatermark? = nil,
+        isInboundStreamOutboundHalfClosureEnabled: Bool = false,
         connectionInitializer: @escaping NIOHTTP2Handler.ConnectionInitializer,
         inboundStreamInitializer: @escaping NIOHTTP2Handler.StreamInitializer
     ) throws -> EventLoopFuture<(
@@ -544,11 +554,15 @@ extension Channel {
             streamDelegate: streamDelegate,
             streamInboundType: streamInboundType,
             streamOutboundType: streamOutboundType,
+            inboundStreamBackpressureStrategy: inboundStreamBackpressureStrategy,
+            isInboundStreamOutboundHalfClosureEnabled: isInboundStreamOutboundHalfClosureEnabled,
             inboundStreamInitializer: inboundStreamInitializer
         ).flatMap { multiplexer in
             return connectionInitializer(self).flatMapThrowing { _ in
                 let connectionAsyncChannel = try NIOAsyncChannel(
                     synchronouslyWrapping: self,
+                    backpressureStrategy: connectionBackpressureStrategy,
+                    isOutboundHalfClosureEnabled: connectionIsOutboundHalfClosureEnabled,
                     inboundType: ConnectionInbound.self,
                     outboundType: ConnectionOutbound.self
                 )
@@ -680,6 +694,8 @@ extension ChannelPipeline.SynchronousOperations {
         position: ChannelPipeline.Position = .last,
         streamInboundType: StreamInbound.Type,
         streamOutboundType: StreamOutbound.Type,
+        inboundStreamBackpressureStrategy: NIOAsyncSequenceProducerBackPressureStrategies.HighLowWatermark? = nil,
+        isInboundStreamOutboundHalfClosureEnabled: Bool = false,
         inboundStreamInitializer: @escaping NIOHTTP2Handler.StreamInitializer
     ) throws -> NIOHTTP2Handler.AsyncStreamMultiplexer<NIOAsyncChannel<StreamInbound, StreamOutbound>> {
         return try self.configureAsyncHTTP2Pipeline(