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?

首先跟踪 asyncpg 的连接设置，并比较 issue 中链接的 socketFactory、DialFunc 和 PyMySQL 参考。确定现有 socket 和可选的 startTLS 应如何适配 API，然后通过涵盖两种 TLS 选项和隧道连接的 coverage 验证完成情况。