Replace DICOMweb test services in pixl_core unit tests with mocks

[milanmlft]

Definition of Done / Acceptance Criteria

The orthanc and dicomweb-server services currently defined in pixl_core/tests/docker-compose.yml have been removed and replaced by mock services in the unit tests.

Testing

• the current pixl_core/uploader/test_dicomweb.py tests should still pass

Documentation

• Document mock services using appropriate docstrings
• Update pixl_core/README.md

Dependencies

No response

Details and Comments

The test services were added in #379 but are duplicated across pixl_core/ and the system test (test/), which is not ideal. In addition, having to spin up the Docker containers to run the pixl_core unit tests is a pain. Ideally we'd want to spin up the services only for the system test.