Refactor crtb and prtb manager to make it more testable

[JonCrowther]

Summary

As it currently stands, there are no unit tests for crtb_handler.go or prtb_handler.go. Part of the reason for that is that the code is set up in a way that is very difficult to test. This PR is a potential refactor that makes the code much more testable. It is important to note that this is just a code refactor, with no changes to functionality.

Commits

I tried as best I could to split the commits into logical blocks to help reviewers

• Commit 1: Move resource clients from manager.go to crtb_handler.go. That way, the manager struct is only referenced to access it's functions, not it's fields.
• Commit 2: Same as 1, but for prtb_handler.go
• Commit 3: Now that several of the clients have been moved out of manager.go, remove any unused clients from the manager struct
• Commit 4: Create a managerInterface that contains all the functions used by crtb_handler.go
• Commit 5: Same as 4, but for prtb_handler.go
• Commit 6: Add an example of a test for crtb_handler.go where we mock the managerInterface
• Commit 7: Use a mock generator for manager and added almost full test coverage of reconcileBindings
• Commit 8: All of the above, but for managementuser

Future work:

• Some of the manager functions really don't need to be member functions, which would simplify the mock. Often these functions reference a single field from manager, which we could pass in when the function is called instead.

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DO NOT MERGE

This is a for example purposes only. After discussion it may get merged.

[andreas-kupries]

Thank you for making the effort here to make the system more testable.
I like the approach very much. Especially as it means that only the high-level
functions need mocking instead of deeper details. This makes tests much simpler.

The only thing I am missing is the equivalent work for the
pkg/controllers/managementuser/rbac/[cp]rtb_handler.go.

That however is something I could do as part of my work on the CRTB status field.

I wholeheartedly approve this PR.
Despite effectively undoing the test work I did for the status field.

[andreas-kupries]

Approving again.

[crobby]

a big improvement, lgtm