Add socket_options to httpx.HTTPTransport.

[tomchristie]

Since version 0.17.2 the httpcore package has support for a socket_options parameter.
We should add a corresponding parameter to httpx.HTTPTransport, which can be passed through to the connection pool.

This will resolve the use-case raised in discussion #2635...

>>> import httpx
>>> socket_options = [(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"ETH999")]
>>> transport = httpx.HTTPTransport(socket_options=socket_options)
>>> client = httpx.Client(transport=transport)