Add the HTTP/2 connection (#1859)

Motivation:

To build up a subchannel we need the notion of a connection to a
backend as a building block. The connection provides a single HTTP/2
connection to the remote peer and doesn't deal with backoff or
reconnects.

Modifications:

- Add the 'Connection' object which provides multiplexed streams to a
  connected backend
- Add a 'connector' API which provides a NIO channel and a multiplexer
  on which streams can be created
- Add test Utilities and tests

Result:

Can create a connection to a backend and run streams on it.

Sources/GRPCHTTP2Core/Client/Connection/ClientConnectionHandler.swift

@@ -18,8 +18,9 @@ import NIOCore
 import NIOHTTP2
 
 /// An event which happens on a client's HTTP/2 connection.
-enum ClientConnectionEvent: Sendable, Hashable {
-  enum CloseReason: Sendable, Hashable {
+@_spi(Package)
+public enum ClientConnectionEvent: Sendable, Hashable {
+  public enum CloseReason: Sendable, Hashable {
     /// The server sent a GOAWAY frame to the client.
     case goAway(HTTP2ErrorCode, String)
     /// The keep alive timer fired and subsequently timed out.