Plugins Implementation

[tconley1428]

What was changed

Add plugin types for Connection, NativeConnection, Worker, Client, and Bundler, as well as a SimplePlugin class for easy construction of new plugins.

Why?

Plugins make it easier to share common configurations and apply them consistently.

Checklist

1. Closes [Feature Request] Plugin support #1764

2. How was this tested:

3. Any docs updates needed?

[chris-olszewski]

Overall, this design makes sense to me.

[mjameswh]

Please make sure to add @experimental on all new APIs introduced in this PR.

I have a few uncertainties regarding the viability of some properties in simple plugins, that will only become obvious once we get to try and implement various actual plugins, so some further API changes are actually probable.