Implement support for SOCKS5 proxies. Implement a new proxy wrapper
that handles SOCKS5 connection, authentication and requesting
connections to the actual Kafka broker endpoints.

The proxy can be configured via a new keyword argument `socks5_proxy`
to consumers, producers or admin client. The value is URL with optional
username and password. E.g.
`socks5://user:secret@proxy.example.com:10800`

The implementation is done in state machine that makes progress on
repeated calls to connect_ex. The rationale with this bit strange
design is to minimize amount of changes on the actual BrokerConnection
object.