[CIVIS-2892] datadog_cov native extension for per test code coverage

[anmarchenko]

What does this PR do? Adds datadog_cov native extension for per test code coverage.

Usage of Datadog::CI::ITR::Coverage::DDCov:

# track files coverage in the current folder
cov = Datadog::CI::ITR::Coverage::DDCov.new(root: Dir.pwd)
cov.start_coverage
Calculator.new.add(1, 2) # => 3
cov.stop_coverage # => { "/path/to/current-folder/calculator.rb" => true }

# track lines coverage
cov = Datadog::CI::ITR::Coverage::DDCov.new(root: Dir.pwd, mode: :lines)
cov.start_coverage
Calculator.new.add(1, 2) # => 3
cov.stop_coverage # => { "/path/to/current-folder/calculator.rb" => {12 => true, 13 => true} }

Additional Notes Current limitations:

• UTF-8 in source file paths is not supported because of additional overhead of parsing UTF-8 strings: will be added later as an opt-in via environment variable as non-ASCII characters in paths and filenames are rare
• Lines coverage is barebones and not optimized: ITR will use only files coverage in the first stage

How to test the change? Unit tests are provided in spec/datadog/ci/cov_spec.rb

Scenarios tested:

• coverage with different root values
• root folder path is longer than covered files' paths
• coverage is not collected after stop_coverage call
• coverage of mixed in code from helpers
• multi threaded coverage (coverage is collected per thread)
• UTF-8 in file names does not fail (but produces incorrect result)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.57447% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.22%. Comparing base (6a994b6) to head (e4e86f3).

Files	Patch %	Lines
lib/datadog/ci/utils/git.rb	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.0 #137 +/- ## ========================================== + Coverage 99.19% 99.22% +0.02% ========================================== Files 163 171 +8 Lines 7240 7468 +228 Branches 302 310 +8 ========================================== + Hits 7182 7410 +228 Misses 58 58 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[anmarchenko]

This pull request is now rebased on top of 1.0 branch and the base is set to 1.0.

From now on, all ITR functionality will go into 1.0 branch so that we don't add additional risks for ddtrace 1.x gem.

[anmarchenko]

@ivoanjo this is ready for another pass