[CIVIS-2934] Request skippable tests when configuring ITR

[anmarchenko]

What does this PR do?

• Adds an API client for fetching skippable tests from ITR backend and calls it when configuring ITR runner.
• Adds os.version tag for tests (ATTENTION: it will change all test fingerprints when 1.0 is released)
• Adds retries mechanism on network errors (unstable DNS resolution of DD staging host inspired me to do that)

How to test the change?
Tested locally using my sidekiq fork and unit tests are provided. For now it saves a set of skippable tests without actually skipping them: to do in the next PR.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (9641d3a) to head (b0ea212).

Additional details and impacted files

@@           Coverage Diff           @@
##              1.0     #159   +/-   ##
=======================================
  Coverage   99.05%   99.05%           
=======================================
  Files         195      195           
  Lines        8957     9032   +75     
  Branches      387      390    +3     
=======================================
+ Hits         8872     8947   +75     
  Misses         85       85           

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[romainkomorndatadog]

(I did not actually have a comment.)

[anmarchenko]

@romainkomorndatadog I will gradually add parameters support in the next PR where test skipping will actually happen