Motivation:

In preparation for exposing APIs which surface HTTP/2 connections and
streams using structured concurrency this PR introduces a store for
inbound HTTP/2 streams.

Modifications:

Define types and methods for storing generic types which wrap `Channels`
corresponding to inbound HTTP/2 streams in an async sequence.

Result:

The new types are not yet exposed, this work introduces part of the
framework for future functionality.

Motivation:

This PR is the first step in exposing APIs which surface HTTP/2 connections and
streams using structured concurrency. This PR exposes the most abstract
spelling of this concept, assuming no particular types/forms for the
types involved in the stream channel types.

Modifications:

* Store a generic `streamInitializer` on the common inbound initializer
  which is used to initialize inbound streams before yielding them to
the continuation of streams.
* Expose pipeline configuration functions which assume HTTP/2 but
  nothing else about sream channel types.

Result:

Adopters of the new SPI should be able to create outbound and deal with
inbound HTTP/2 stream channels using async streams.

Outside of SPI there should be no changes.

* Finish stream channel sequence continuation
* Provide internal functions for creating streams without configuring
  them. Configuring leads to activation so it can be helpful to allow
that step to be performed manually once any provided initialization
closures have beenn executed.