feat: allow connection with pre-configured socket

Nobody has claimed this yet.

I'd like to add support for creating a postgres connection over an existing socket like object. In this approach startTLS can or cannot be used depending on the caller. This lines up similarly with what we have in Java ([socketFactory connection param](https://jdbc.postgresql.org/documentation/use/#connection-parameters), [driver code](https://github.com/pgjdbc/pgjdbc/blob/aff581fe806f1c1cdc8bcd9e5a981e37691b44b5/pgjdbc/src/main/java/org/postgresql/core/SocketFactoryFactory.java#L25-L47)), Go ([pgx DialFunc](https://pkg.go.dev/github.com/jackc/pgconn#DialFunc)) (these two are slightly different as they allow specifying a creator/generator func that creates the socket, while here we could just pass in the socket directly but the extent of the feat is the same), and other Python libraries ([pymysql](https://github.com/PyMySQL/PyMySQL/blob/main/pymysql/connections.py#L626), and [pg8000](https://github.com/tlocke/pg8000/pull/136))

The equivalent pymysql PR that introduced this change explains the feature really well https://github.com/PyMySQL/PyMySQL/pull/355

The benefit of this feature is that it allows the user to specify their own secure tunnel to connect over (such as ssh).

Is this sort of feat possible for asyncpg?

Start by tracing asyncpg's connection setup and compare the socketFactory, DialFunc, and PyMySQL references linked in the issue. Determine how an existing socket and optional startTLS would fit the API, then verify completion with coverage for both TLS choices and tunneled connections.

Written by the indexing model from the issue text.