Impersonation client

[KevinJoiner]

Add a WithImpersonation option for clients that allows users to get a client set up for impersonation.

Notable Changes

• I added the concept of client Mutators to reduce duplicate code for controllers and factories.
  • the user_agent test code was not deleted but moved into mutator_test.go
• WithAgent and WithImpersonation now update the returned config so that both options can be used together.
• SharedControllerWithMutator mutates the client once on construction and returns the stored client with every call to Client() instead of generating a new client everytime to mimick the same behavior of SharedController.

[MbolotSuse]

I like the general pattern, and think that making this more generic was a smart move. Good job on that.

Thank you for adding comments as well.

Two overall suggestions:

• I'd recommend for allowing for more than one mutator. I've left a comment on the client about this (would also apply to the controllers) and what it would look like. It does change the exported functions here, so I'm open to discussion on if that is useful or not.
• In the future (probably not part of this PR), we should consider using code-gen/generics so that each new mutator doesn't result in changes to clientMutator and controllerMutator to expose the new NewSharedFactoryWithMutator option. We don't have to do this here, but it's something to think about.

[MbolotSuse]

Actual code LGTM. Have a couple of comments (mostly nits) on the tests.

Non-nit test comments:

• We should probably test WithAgent and WithImpersonation on the client directly, particularly since we can access the config and need to assert that it's not overwriting the previously-set options afterwards
• The tests for Watch ignore the watch.Interface return. Since this is the part that is actually returning objects to us, I think we should make some type of test/assertion which accounts for it.

[rmweir]

I remember you mentioning this idea:
SharedControllerWithMutator mutates the client once on construction and returns the stored client with every call to Client() instead of generating a new client everytime to mimick the same behavior of SharedController.

but I don't think I see it implemented anywhere, am I missing something?

[rmweir]

Suggested change

	// Mutator is function that takes in a client and returns an new mutated client.
	// Mutator is function that takes in a client and returns a new mutated client.

[KevinJoiner]

@rmweir Originally, I had added mutators and impersonation to the SharedControllerFactory but ultimately decided against it, so I left the code that creates a new client as is.

I decided not to add impersonation to the Controllers because in practice we would not want an end user to create a ControllerFactory with impersonation. Instead, we would want to keep it scoped to the client level for one-off situations.