Allow passing a local address and address family to trio.open_tcp_stream

[ntninja]

Given a DNS-name trio.open_tcp_stream currently has no way to limit the TCP connection to either IPv4 or IPv6 and has no way to bind the local socket to a specific interface or port. While this is not an issue in general and trio's native happy-eyeballs support is awesome, there are some advanced use cases where being more restrictive can be useful.

To that point there is currently a patch proposed for httpcore/httpx that allows overriding the local_addr and family parameters globally for a connection pool. It's missing trio support however as trio does not support the require parameters.

[njsmith]

#275 is our canonical issue for discussing this feature – let's continue conversation there.