Allow socket options to be configured.

[tomchristie]

We should add a socket_options configuration to the ConnectionPool and HTTPProxy classes.

This would need to be passed through to the network backends connect_tcp(...) and connect_unix_socket() methods.

For example the sync case, here...

httpcore/httpcore/backends/sync.py

Lines 80 to 97 in 21450a7

	def connect_tcp(
	self,
	host: str,
	port: int,
	timeout: typing.Optional[float] = None,
	local_address: typing.Optional[str] = None,
	) -> NetworkStream:
	address = (host, port)
	source_address = None if local_address is None else (local_address, 0)
	exc_map: ExceptionMapping = {
	socket.timeout: ConnectTimeout,
	OSError: ConnectError,
	}
	with map_exceptions(exc_map):
	sock = socket.create_connection(
	address, timeout, source_address=source_address
	)
	return SyncStream(sock)

...which we would adapt to something like this...

def connect_tcp( 
    self, 
    host: str, 
    port: int, 
    timeout: typing.Optional[float] = None, 
    local_address: typing.Optional[str] = None,
    socket_options: typing.Sequence[typing.Tuple[int, int, typing.Union[int, bytes]]] = ()
 ) -> NetworkStream: 
    address = (host, port) 
    source_address = None if local_address is None else (local_address, 0) 
    exc_map: ExceptionMapping = { 
        socket.timeout: ConnectTimeout, 
        OSError: ConnectError, 
    }
    with map_exceptions(exc_map): 
        sock = socket.create_connection( 
            address, timeout, source_address=source_address 
        )

    # See https://docs.python.org/3/library/socket.html#socket.socket.setsockopt
    for option in socket_options:
        sock.setsockopt(*option)

    return SyncStream(sock) 

This would resolve #651 and encode/httpx#2635

We could also consider changing our defaults for TCP_NODELAY, but we should consider that option as a follow-up.

[tomchristie]

We'd also want to then push this configuration through to httpx.HTTPTransport.