feat: allow connection with pre-configured socket

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, driver code), Go (pgx 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, and pg8000)

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?

Beginne damit, asyncpgs Verbindungsaufbau nachzuverfolgen und die im Issue verlinkten Referenzen zu socketFactory, DialFunc und PyMySQL zu vergleichen. Ermittle, wie ein vorhandener Socket und optionales startTLS in die API passen würden, und überprüfe den Abschluss mit Testabdeckung für beide TLS-Optionen und getunnelte Verbindungen.