[Cases] Create package for mocks

[cnasikas]

In PR #151761 we needed to mock the Case UI client in the triggers_actions_ui plugin. Cases require and use the triggers_actions_ui plugin. To avoid circular dependencies we duplicate the code. We should create a package that contains the required mocks so they can be imported to other plugins and avoid circular dependencies if cases depend on them.

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)