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

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* **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](https://github.com/magicstack/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](https://github.com/MagicStack/asyncpg/blob/c2c8d20ba8c0057b0e0bc08bd892abc36862e553/asyncpg/pool.py#L1088-L1094) 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](https://docs.sqlalchemy.org/en/20/core/events.html#sqlalchemy.events.DialectEvents.do_connect) or [pgx](https://github.com/jackc/pgx/blob/5c63f646f820ca9696fc3515c1caf2a557d562e5/pgxpool/pool.go#L107-L109).

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.

Start with asyncpg/pool.py, especially the existing init callback and the acquire/set_connect_args paths described in the issue, to understand when connections are created versus checked out. Done means a before-connect hook can refresh dynamic credentials only when a new pool connection is created, rather than on every acquire.

Written by the indexing model from the issue text.