Remove exported mocks

[toddbaert]

Exported, generated mocks should be deleted. I don't see the need to have these exported if their purpose is testing for this SDK. Example: https://github.com/open-feature/go-sdk/blob/main/openfeature/hooks_mock.go

Remove exported generated mocks. These are for internal testing.

If there is real testing utils we want to export, we should put them in a /testing package.

[jblacker]

I wrote this under #412 since I was specifying where to move the mocks. Here's what I posted there to keep the discussion on-topic:

Personally, I don't think we should unexport the mocks, but I do believe that they should be relocated. Once we break up the interfaces more I think we should regenerate them into the testing package to allow them to be optional. The reason for this is that they can be quite useful when writing tests for custom providers & hooks. I know there are other mocking frameworks out there and not everyone uses ubermock though. Maybe another option would be to put them into their own package nested under testing like testing/mocks. This would allow for usage internally as well as still allowing provider & hook developers to leverage them.