feat: add connect_with_socket() for custom transports

[airhorns]

I want to make use of the turmoil crate to implement deterministic simulation testing for a system I'm working on that makes use of sqlx. Turmoil works by virtualizing all the networking that different elements of a system do so it can inject faults, like in my app's connection to MySQL! sqlx doesn't need to know anything about turmoil, but before this change, it wasn't possible to inject in turmoil's special socket implementation that lets it do its thing. It's weird to say, but sqlx "hardcoded" the use of tokio's socket.

To let folks like me inject alternative implementations of the Socket trait, we need a new high level public function for connecting using our own sockets! This adds that. The same api also would let folks inject sockets that do weird other transport stuff, like tunnels or proxies.

Changes:

• Add net::connect_with() in sqlx-core as the analog of connect_tcp
  and connect_uds for pre-established sockets
• Add MySqlConnectOptions::connect_with_socket() and
  MySqlConnection::connect_with_socket() for MySQL
• Add PgConnectOptions::connect_with_socket() and
  PgConnection::connect_with_socket() for PostgreSQL
• Re-export Socket trait and connect_with from top-level sqlx::net
• Refactor MySQL post-connect init into shared after_connect() to
  avoid duplication between connect() and connect_with_socket()

The Socket trait was already public; these changes simply expose a way to pass a pre-connected Socket into the connection establishment flow that was previously only reachable through connect_tcp/connect_uds.

Does your PR solve an issue?

No, no open issue

Is this a breaking change?

No, no breaking change

[abonander]

Duplicate of #4187