Dedicated job serialiser for event logging

[Eldies]

Motivation and context

JobReadSerializer has a field issues. The field is only filled when jobs are requested in API.
When update handler for jobs is triggered, the field is empty for old job instance.
Due to that, if the job has X issues, every update to the job, even if it does not involve issues, creates a server event which incorrectly says that number of issues changed from 0 to X.

Since the only way to fix the value is to introduce an additional join on every update, and since there exists another way to get changes of issues (their own create/delete events), the best way to deal with the problem is to split JobReadSerializer into two serialisers: JobReadForEventSerializer for logging events and JobReadSerializer(JobReadForEventSerializer) which is used in API

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.91%. Comparing base (e9fae7a) to head (1f873f3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #10257      +/-   ##
===========================================
+ Coverage    75.90%   75.91%   +0.01%     
===========================================
  Files          430      430              
  Lines        46573    46578       +5     
  Branches      4198     4198              
===========================================
+ Hits         35352    35362      +10     
+ Misses       11221    11216       -5     

Components	Coverage Δ
cvat-ui	77.68% <ø> (+<0.01%)	⬆️
cvat-server	74.41% <100.00%> (+0.02%)	⬆️

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