Add MockServer container

[Xeonian]

Add a MockServer container class.

Uses the mockserver-client library to supply a client.

Had to pin grpcio-status dependency for the google-cloud-pubsub extra as the latest version produces conflicting requirements for the protobuf dependency.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.33%. Comparing base (472da35) to head (0cc2fc8).
Report is 301 commits behind head on main.

❗ Current head 0cc2fc8 differs from pull request most recent head 4e6890b. Consider uploading reports for the commit 4e6890b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
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+ Coverage   85.93%   86.33%   +0.40%     
==========================================
  Files          28       29       +1     
  Lines         782      805      +23     
  Branches       77       78       +1     
==========================================
+ Hits          672      695      +23     
  Misses         81       81              
  Partials       29       29              

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[pffijt]

Would it be better to make use of the port argument so one can overwrite the port?
Since the port can be changed with an environment variable: https://www.mock-server.com/where/docker.html#docker_container_environment_variables

[pffijt]

I think it would be better to document what the example does?