[SDTEST-786] support logical names for test sessions

[anmarchenko]

What does this PR do?
Adds support for the logical test session names for test visibility.

Now users are able to name their test sessions using DD_TEST_SESSION_NAME variable. If the environment variable is not provided, "#{ci_job_name}-#{test_command}" is used as default.

Motivation
Currently test sessions suffer from unstable fingerprints issue: we use the full test command as the test session identifier right now, and test command might have some random bits (like seed for example). This makes every test session to be considered new by the Datadog's backend, and breaks some features - for example, ITR savings calculation.

This is the first step to migrate to the new definition of test session name.

How to test the change?
Unit tests are provided

Tested using sidekiq project on production:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.86364% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.95%. Comparing base (0fc0203) to head (96c32c7).

Files with missing lines	Patch %	Lines
lib/datadog/ci/test_visibility/null_component.rb	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
- Coverage   98.95%   98.95%   -0.01%     
==========================================
  Files         273      273              
  Lines       13285    13368      +83     
  Branches      597      600       +3     
==========================================
+ Hits        13146    13228      +82     
- Misses        139      140       +1     

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