[CIVIS-2892] citestcov transport to serialize and send code coverage events

[anmarchenko]

What does this PR do?

• Adds citestcov_request method to Transport::Api::Base to send events to citestcov intake as multipart/form-data (via agent or agentless intake)
• Adds Coverage::Event class that serializes itself to msgpack
• Adds Coverage::Transport class that packages msgpack payload and sends it to the intake
• Deduplicated code in Coverage::Transport and TestVisibility::Transport by introducing common ancestor EventPlatformTransport
• Fixed regression caused by a breaking change in datadog gem v2.0 with Datadog::Core::Transport::HTTP::Adapters::Net initialization

How to test the change?
Unit tests are provided

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.63899% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.23%. Comparing base (61be62e) to head (4ffbcb5).

Files	Patch %	Lines
lib/datadog/ci/itr/coverage/transport.rb	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #148      +/-   ##
==========================================
+ Coverage   99.22%   99.23%   +0.01%     
==========================================
  Files         171      175       +4     
  Lines        7468     7730     +262     
  Branches      310      317       +7     
==========================================
+ Hits         7410     7671     +261     
- Misses         58       59       +1     

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