Introduce carriers for HttpClient transports

[sbordet]

Jetty version
10.0.x

Description
Java 16 introduced UnixDomain support (#6043).
HTTP3 over QUIC is another option.

We need a way to "carry" protocol-specific bytes (e.g. HTTP/1.1 bytes or HTTP/2 bytes or HTTP/3 bytes) using different "carriers", i.e. via TCP, UnixDomain, or QUIC (over UDP).

A proposal is the following:

ClientConnector clientConnector = new CompositeClientConnector();
clientConnector.add(new UnixDomainClientConnector("/path8080", "localhost:*"));
clientConnector.add(new QUICClientConnector("quic.cloudflare.com:443"));
clientConnector.add(new TCPClientConnector("*"));

HttpClient httpClient = new HttpClient(new HttpClientTransportDynamic(clientConnector));
httpClient.start();

The idea is that when we need to open a new carrier endpoint, we call ClientConnector.connect(SocketAddress, Map).
The CompositeClientConnector will do a match on the carriers it manages to see which one matches best (so there must be some "weight" or partial ordering to pick one among many that match).

In this way, all the other concepts will work (e.g. SOCKS4 proxying, as well as HTTP proxying) because they will be orthogonal to the carriers.
Only at the moment that you have to physically establish a carrier endpoing, we go to the ClientConnector, that will decide what type of carrier creates the endpoint.

[sbordet]

Closing as this mechanism has been incorporated in the work done in #6765.