Make AsyncpgConfig generic to support user defined record classes

[iyad-f]

Currently the get_connection method returns AsyncGenerator[AsyncpgConnection, None], which wont be the correct type for all users, since a lot of users have custom defined record classes, so imo we should make AsyncpgConfig accept a Record generic which we will use to type hint generators which are yielding the connection. If you are okay with this i can go ahead and make a PR

[iyad-f]

If we do this then there is no longer a requirement for AsyncpgConnection type alias, but it is currently public so removing it will be a breaking change so what should we do about that?

[iyad-f]

@cofin Can you take a look at this? I have already made the changes and those can be PR'd anytime, just want your confirmation and what should we do about AsyncpgConnection. I am working on a litestar-backend-template and if this gets implemented it will save me all the type ignores where i manually use the get_connection method of the AsyncpgConfig.