feat(go): add initial Go module

[ebaron]

This contains the initial Golang code and tests ported over from the Cryostat Operator. The API to be consumed as a Go module is moved to pkg/insights, while the internal bits stay under internal/controllers.

There will be additional files based on the Operator SDK that will also make this consumable as a container image, but I'll propose that in a separate PR. The Makefile contains some Operator SDK boilerplate that doesn't make sense in the context of this PR, but I included it since the Makefile provides the setup necessary to run the tests with EnvTest. To run them, use make test.

[csviri]

Added some minor comments. Just one another nitpick, it would be nice to add label selectors (and labels) for informers and objects, so in case of large cluster not every object is cached.