Refactor credential refresh

[lukebakken]

Fixes #1639

• Remove all traces of credential refresh from RabbitMQ.Client
• Add CredentialsRefresher, which does not use timers, and calls refresh based on ValidUntil

Note, there is still a lot of work to get all tests running, but this gives you the idea.

Also, there is a big TODO around integrating these changes with PlainMechanism

cc @danielmarbach @MatthiasWerning

[Tornhoof]

This is mostly stuff, I noticed when I experimented making the current api async.

I know you're still working on it, but anyway:

[lukebakken]

@Tornhoof I appreciate you taking a look!

[danielmarbach]

What @Tornhoof said

[lukebakken]

@danielmarbach @MatthiasWerning @Tornhoof - I'm pretty much done with this. I'm going to modify the OAuth2 integration test to include restarting RabbitMQ mid-test, and probably add those annotations to JsonToken.

[danielmarbach]

@lukebakken before I take a deeper look I'm curious to hear why you didn't go down the route of collapsing the refreshing into the provider similar to how Azure.Identity does it?

[lukebakken]

before I take a deeper look I'm curious to hear why you didn't go down the route of collapsing the refreshing into the provider similar to how Azure.Identity does it?

Time constraints.

[Tornhoof]

This looks a lot better than before :)

[lukebakken]

@danielmarbach @Tornhoof @paulomorgado @MatthiasWerning @MarcialRosales

I plan on merging this in a few hours to produce another version 7 RC.

[danielmarbach]

The earliest I can review is probably tomorrow

[lukebakken]

Thanks @danielmarbach. I'm going to merge this PR to produce a new RC, but of course I welcome your review. Any changes can be addressed in a follow-up PR.