[CIVIS-2844] Add test session public API

[anmarchenko]

What does this PR do?
First step towards achieving test suite level visibility (TSLV) in Ruby

Adds test session manual API that is used like that:

Datadog::CI.start_test_session(
  service: "my-web-site-tests",
  tags: { Datadog::CI::Ext::Test::TAG_FRAMEWORK => "my-test-framework" }
)

# .....

Datadog::CI.active_test_session.finish

On recorder level it adds:

• Datadog::CI::TestSession domain model that wraps tracer's span that represents the whole session
• Datadog::CI::ConcurrentSpan domain model that provides thread safety around tracer's span (TestSession is inherited from ConcurrentSpan as it is shared between threads)
• Datadog::CI::Context::Global that represents process-global context that will store active test session, active test module, and a set of active test suites

On transport level it adds:

• a new serializer factory Serializers::Factories::TestSuiteLevel that is capable of creating test suite level payload serializers
• a new serializer Serializers::TestV2 that converts test span to citestcycle test event with version 2 (containing test_session_id field)
• a new serializer Serializers::TestSession that converts session span to citestcycle event

To configuration it adds:

• new temporary config value settings.ci.experimental_test_suite_level_visibility_enabled - it will exists only for transitionary period before test suite level visibility in Ruby is going to be tested and adopted

Notes

Issues to solve or test in the future:

• currently all tests are part of a single test session trace when TSLV is enabled - this is not correct as each test must be a separate trace (will be solved in subsequent PR)
• forking behaviour is not properly tested yet, to test using https://github.com/anmarchenko/openstreetmap-website with Rails parallel executor and adapt accordingly

How to test the change?
Code is covered by a set of unit tests, end2end testing will be possible as soon as test modules and test suites are implemented

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (96281ad) 99.20% compared to head (d656b42) 99.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
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+ Coverage   99.20%   99.30%   +0.09%     
==========================================
  Files         111      125      +14     
  Lines        3792     4328     +536     
  Branches      141      153      +12     
==========================================
+ Hits         3762     4298     +536     
  Misses         30       30              

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