Add "debug initializer" hook for channels (#801)

Motivation:

As requested in #596, it can be handy to have a lower-level access to
channels (HTTP/1 connection, HTTP/2 connection, or HTTP/2 stream) to
enable a more fine-grained interaction for, say, observability, testing,
etc.

Modifications:

- Add 3 new properties (`http1_1ConnectionDebugInitializer`,
`http2ConnectionDebugInitializer` and
`http2StreamChannelDebugInitializer`) to `HTTPClient.Configuration` with
access to the respective channels. These properties are of `Optional`
type `@Sendable (Channel) -> EventLoopFuture<Void>` and are called when
creating a connection/stream.

Result:

Provides APIs for a lower-level access to channels.

---------

Co-authored-by: Cory Benfield <lukasa@apple.com>
Co-authored-by: David Nadoba <d_nadoba@apple.com>
Co-authored-by: George Barnett <gbarnett@apple.com>

Sources/AsyncHTTPClient/ConnectionPool/HTTPConnectionPool.swift

@@ -324,7 +324,9 @@ final class HTTPConnectionPool:
             connection.executeRequest(request.req)
 
         case .executeRequests(let requests, let connection):
-            for request in requests { connection.executeRequest(request.req) }
+            for request in requests {
+                connection.executeRequest(request.req)
+            }
 
         case .failRequest(let request, let error):
             request.req.fail(error)