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/HTTPClient.swift

@@ -847,6 +847,15 @@ public class HTTPClient {
         /// By default, don't use it
         public var enableMultipath: Bool
 
+        /// A method with access to the HTTP/1 connection channel that is called when creating the connection.
+        public var http1_1ConnectionDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)?
+
+        /// A method with access to the HTTP/2 connection channel that is called when creating the connection.
+        public var http2ConnectionDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)?
+
+        /// A method with access to the HTTP/2 stream channel that is called when creating the stream.
+        public var http2StreamChannelDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)?
+
         public init(
             tlsConfiguration: TLSConfiguration? = nil,
             redirectConfiguration: RedirectConfiguration? = nil,
@@ -949,6 +958,32 @@ public class HTTPClient {
                 decompression: decompression
             )
         }
+
+        public init(
+            tlsConfiguration: TLSConfiguration? = nil,
+            redirectConfiguration: RedirectConfiguration? = nil,
+            timeout: Timeout = Timeout(),
+            connectionPool: ConnectionPool = ConnectionPool(),
+            proxy: Proxy? = nil,
+            ignoreUncleanSSLShutdown: Bool = false,
+            decompression: Decompression = .disabled,
+            http1_1ConnectionDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)? = nil,
+            http2ConnectionDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)? = nil,
+            http2StreamChannelDebugInitializer: (@Sendable (Channel) -> EventLoopFuture<Void>)? = nil
+        ) {
+            self.init(
+                tlsConfiguration: tlsConfiguration,
+                redirectConfiguration: redirectConfiguration,
+                timeout: timeout,
+                connectionPool: connectionPool,
+                proxy: proxy,
+                ignoreUncleanSSLShutdown: ignoreUncleanSSLShutdown,
+                decompression: decompression
+            )
+            self.http1_1ConnectionDebugInitializer = http1_1ConnectionDebugInitializer
+            self.http2ConnectionDebugInitializer = http2ConnectionDebugInitializer
+            self.http2StreamChannelDebugInitializer = http2StreamChannelDebugInitializer
+        }
     }
 
     /// Specifies how `EventLoopGroup` will be created and establishes lifecycle ownership.