Defer PostgreSQL passfile lookup

[V02460]

Loads the the PostgreSQL passfile on apply_pgpass and defers querying the password to get_password calls. This ensures the correct password is retrieved, even when builder methods are called on PgConnectOptions after initialization.

Does your PR solve an issue?

Fixes #2658

Is this a breaking change?

Yes. Parameters set by PgConnectOptions builder methods are now honored by the passfile lookup.

[abonander]

I don't agree with implicitly attempting to re-apply the .pgpass file after construction and manual modification. I think that's going to be too surprising.

Consider a case like #3832 where the user doesn't even want the libpq behavior to begin with. This would be horrifying to them.

This would also break use-cases that need to implement password rotation, though that's also a really eclectic mix of concerns (.pgpass files shouldn't really be used in production, but if you are then why aren't you just re-reading the .pgpass file).

I think if we want this behavior, we should just expose the PgPassFile type (n.b. PGPassFile doesn't match the code style) and provide a way to apply a PgPassFile instance to an already-constructed PgConnectOptions.