Add typing to _ConnectionParameters and related functions

[DanielNoord]

This adds typing to an important NamedTuple in this module that will allow using types in a lot of other functions that have a reference to this object.

The final type looks very similar to what was suggested in #577 but this is based on the paths that the code can actually take. As with other PRs the strategy is simple: start with the lowest function in the call chain to see if the code can tell you what types it should accept and propagate upwards until you reach the NamedTuple.
The most controversial change is probably _ConnectionParameters.ssl. This is because of the code path where we set sslmode = SSLMode.disable. It is a bit annoying but that code path means that ssl never gets reassigned and therefore will be whatever _parse_connect_dsn_and_args accepts. We should probably change that to be None but I don't want to introduce behaviour changes in these PRs.

The two actual runtime changes are straightforward and any mistakes in the typing we can (of course) always rectify later.

[DanielNoord]

Based on review in #1198 I have swapped out the stringified annotations for non-stringified behind a from __future__ import annotations.

I also had to rebase and force push due to merge conflicts.