Refactoring and comments

Large scale refactoring of code and function visibility to make the
various APIs currently under SPI more manageable.

As the API surface has become more complex with the addition of more
layers it became apparent that we needed to take steps to simplify.

Sources/NIOHTTP2/HTTP2ChannelHandler.swift

@@ -1026,6 +1026,9 @@ extension NIOHTTP2Handler {
     }
 
     /// Connection-level configuration.
+    ///
+    /// The settings that will be used when establishing the connection. These will be sent to the peer as part of the
+    /// handshake.
     public struct ConnectionConfiguration: Hashable, Sendable {
         public var initialSettings: HTTP2Settings = nioDefaultSettings
         public var headerBlockValidation: ValidationState = .enabled
@@ -1036,13 +1039,25 @@ extension NIOHTTP2Handler {
     }
 
     /// Stream-level configuration.
+    ///
+    /// The settings that will be used when establishing new streams. These mainly pertain to flow control.
     public struct StreamConfiguration: Hashable, Sendable {
         public var targetWindowSize: Int = 65535
         public var outboundBufferSizeHighWatermark: Int = 8196
         public var outboundBufferSizeLowWatermark: Int = 4092
         public init() {}
     }
 
+    /// Overall connection and stream-level configuration.
+    public struct Configuration: Hashable, Sendable {
+        /// The settings that will be used when establishing the connection. These will be sent to the peer as part of the
+        /// handshake.
+        public var connection: ConnectionConfiguration = .init()
+        /// The settings that will be used when establishing new streams. These mainly pertain to flow control.
+        public var stream: StreamConfiguration = .init()
+        public init() {}
+    }
+
     /// An `EventLoopFuture` which returns a ``StreamMultiplexer`` which can be used to create new outbound HTTP/2 streams.
     ///
     /// > Note: This is only safe to get if the ``NIOHTTP2Handler`` uses a local multiplexer,