Support a Pool `before_connect` callback to be used before a connection is created

从 asyncpg/pool.py 开始，重点查看现有的 init 回调以及 issue 中描述的 acquire/set_connect_args 路径，以了解连接何时创建、何时从池中取出。完成的标准是，before-connect hook 只能在创建新的池连接时刷新动态凭据，而不是在每次 acquire 时刷新。

• asyncpg version: 0.27.0
• PostgreSQL version: 14
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: Local
• Python version: 3.9.9
• Platform: macos
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?:

Hello! This is more of a feature request but I was wondering if there could be support for having a before_connect or similar callback when creating a Pool object that would run before a new connection is created? I see there is already an init param which is run after a connection is made.

Our use case is that we use dynamic credentials to access our postgres database and want to make sure that when we create a connection we are using credentials that are not stale. Is this something that makes sense for this project? There is similar functionality in other libraries such as sqlalchemy or pgx.

I was looking at how to do this as-is and there were two currently that I was exploring. One way would be to create our own Pool class which seems like a lot of maintainance for what we'd like to do. The other looked at creating a subclass and overriding the acquire and using set_connect_args but that would run every time we get a connection from a pool which would be wasteful as opposed to just when the creation happens.