Adding support for transform functions

[MbolotSuse]

Related to one of the tasks from rancher/rancher#45635. Adds the option for a TransformFunc which will be can be passed in when the Cache is created.

[tomleb]

I don't see any tests that uses a TransformFunc. Can that be added so we have some confidence that we're calling SetTransform, etc?

[moio]

Agreed with @tomleb: this is good to merge as soon as at least one test touches the newly added parameter.

[MbolotSuse]

@tomleb / @moio I'll see what I can do. Right now the tests are a bit mock-reliant, and since we can't test functions for equality (see issues like: golang/mock#324) there may be a limit to what I can do.

[MbolotSuse]

@moio / @tomleb I gave it a try. I had to do a few sub-optimal/hacky things:

• I had to move the cached.SharedIndexInformer call to a "newInformer" call.
  • The SharedInformer seems to have a method to set the transformer, but no method to get the transformer afterward. So it was either this or do some sort of unsafe get to pull the method out after create.
• I can't use mockgen to create a SharedIndexInformer since we use UnsafeSet to override the indexer (which won't exist on a mock).
  • This required that I manually make quick mock to save the transform function
• Since I can't (afaik) compare function equality, I need to call the function that ends up stored on the mock informer to make sure that it's the same function that we set before.

Please take a look and let me know if you have some other ideas (or if you would prefer that I not add the tests in favor of the first version of the code).