[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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.