[SDTEST-60] Report total lines coverage percentage to Datadog

[anmarchenko]

What does this PR do?
Adds support for total code coverage reporting feature.

Supported code coverage library: gem "simplecov" >= 0.18.0

Motivation
Achieving feature parity across Datadog libraries

How to test the change?

Tested using rubocop project:

... and middleman:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.89899% with 10 lines in your changes missing coverage. Please review.

Project coverage is 98.89%. Comparing base (17b2e4f) to head (9b12d3d).

Files with missing lines	Patch %	Lines
lib/datadog/ci/test_visibility/total_coverage.rb	60.00%	8 Missing ⚠️
...b/datadog/ci/contrib/simplecov/result_extractor.rb	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
- Coverage   98.95%   98.89%   -0.07%     
==========================================
  Files         273      279       +6     
  Lines       13425    13524      +99     
  Branches      609      620      +11     
==========================================
+ Hits        13285    13374      +89     
- Misses        140      150      +10     

Flag	Coverage Δ
	98.89% <89.89%> (-0.07%)	⬇️

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