[CIVIS-2938] Git upload async worker

[anmarchenko]

What does this PR do?
Finishes git metadata upload feature:

• add Datadog::CI::Worker for general purpose async tasks execution
• creates worker instance with a task to upload git metadata and supplies it to Datadog::CI::Recorder via constructor
• Datadog::CI::Recorder starts worker when test session is started
• added configuration option DD_CIVISIBILITY_GIT_METADATA_UPLOAD_ENABLED to opt-out of this functionality if needed

How to test the change?
Tested using staging and local sidekiq project configured with CI visibility. Unit tests are provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (39f3f3c) to head (9fab5aa).

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #156      +/-   ##
==========================================
+ Coverage   99.00%   99.04%   +0.03%     
==========================================
  Files         190      192       +2     
  Lines        8665     8786     +121     
  Branches      371      379       +8     
==========================================
+ Hits         8579     8702     +123     
+ Misses         86       84       -2     

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