Add unit tests for object model

[leonhard-s]

With the internal restructuring going on in the models branch right now (065a0e9c and following), unit tests for the object model should be added before merging to ensure compatibility.

Housekeeping:

• [x] Update the existing test cases to ensure full coverage:
  • [x] cache_test.py
  • [x] census_test.py
  • [x] query_test.py
• [x] Set up a custom service ID for repository CI/CD as not all things can be tested offline

Object model tests:

• [x] Add a separate set of tests for the upcoming models submodule. These will be run regularly to ensure the object model matches the API, without having to wait for users to find errors.
• [x] Add event streaming test cases
• [ ] Add proxy system test cases
• [x] Add online test cases for common operations. This will require some slow-down to be a good API citizen.

Bonus points:

• [x] Add a test coverage checker through GitHub actions
• [x] Add a test coverage status badge

[leonhard-s]

Skipped the proxy system test cases as I am not sure if it is a system worth keeping down the road. This will be tracked in an upcoming issue.