[CIVIS-175] refactor env providers to support custom logic to determine which env they handle

[anmarchenko]

What does this PR do?
Changes the env provider selection logic and pushes it down to specific provider classes in order to support custom logic for selection (like when the ENV value must start with specific prefix).

Motivation
Preparation for AWS CodePipeline support

How to test the change?
Unit tests

[codecov-commenter]

Codecov Report

Merging #52 (1afdbdd) into main (9b83a30) will decrease coverage by 0.03%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   99.15%   99.13%   -0.03%     
==========================================
  Files          95       95              
  Lines        3324     3352      +28     
  Branches      129      129              
==========================================
+ Hits         3296     3323      +27     
- Misses         28       29       +1     

Files	Coverage Δ
lib/datadog/ci/ext/environment/providers.rb	100.00% <100.00%> (ø)
...b/datadog/ci/ext/environment/providers/appveyor.rb	100.00% <100.00%> (ø)
lib/datadog/ci/ext/environment/providers/azure.rb	100.00% <100.00%> (ø)
.../datadog/ci/ext/environment/providers/bitbucket.rb	100.00% <100.00%> (ø)
...ib/datadog/ci/ext/environment/providers/bitrise.rb	100.00% <100.00%> (ø)
lib/datadog/ci/ext/environment/providers/buddy.rb	100.00% <100.00%> (ø)
.../datadog/ci/ext/environment/providers/buildkite.rb	100.00% <100.00%> (ø)
...b/datadog/ci/ext/environment/providers/circleci.rb	100.00% <100.00%> (ø)
.../datadog/ci/ext/environment/providers/codefresh.rb	100.00% <100.00%> (ø)
...dog/ci/ext/environment/providers/github_actions.rb	100.00% <100.00%> (ø)
... and 5 more

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