[CIVIS-2892] Code coverage events writer

[anmarchenko]

What does this PR do?
Adds Datadog::CI::ITR::Coverage::Writer that asynchronously sends coverage events to backend.
It starts a worker thread that is later gracefully stopped in ITR::Runner#shutdown! method.

Unit tests are provided

How to test the change?
Tested using anmarchenko/sidekiq project and executing the following DDSQL queries:

Unit tests are provided. Note that Codecov reports 12 uncovered lines from writer_spec.rb file: this is due to simplecov missing coverage by forked processes.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.03106% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 99.03%. Comparing base (e73065c) to head (4ae8ab7).

Files	Patch %	Lines
spec/datadog/ci/itr/coverage/writer_spec.rb	92.40%	12 Missing ⚠️
lib/datadog/ci/itr/coverage/writer.rb	93.87%	3 Missing ⚠️
...tadog/ci/contrib/rspec/coverage_collection_spec.rb	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #150      +/-   ##
==========================================
- Coverage   99.19%   99.03%   -0.17%     
==========================================
  Files         176      179       +3     
  Lines        7741     8049     +308     
  Branches      314      333      +19     
==========================================
+ Hits         7679     7971     +292     
- Misses         62       78      +16     

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