feat: Add GitHub Actions Resource Detector

[smithclay]

Which problem is this PR solving?

• Adds a new resource detector for GitHub Actions runner environments. This helps developers collect telemetry in GitHub CI/CD environments.

Originally a PR open-telemetry/opentelemetry-js#1681, moved to this repo after discussion.

Short description of the changes

• Detects GitHub-specific environment variables (i.e. GITHUB_SHA, GITHUB_REF) in Action runners and adds them as resource attributes if they exist.
• Adds new directory structure for resource detectors.

[codecov]

Codecov Report

Merging #265 (d3dcada) into master (f039f1f) will increase coverage by 0.01%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   94.94%   94.95%   +0.01%     
==========================================
  Files         110      115       +5     
  Lines        5937     5990      +53     
  Branches      617      618       +1     
==========================================
+ Hits         5637     5688      +51     
- Misses        300      302       +2     

Impacted Files	Coverage Δ
...pentelemetry-resource-detector-github/.eslintrc.js	0.00% <0.00%> (ø)
...ntelemetry-resource-detector-github/src/version.ts	0.00% <0.00%> (ø)
...ce-detector-github/src/detectors/GitHubDetector.ts	100.00% <100.00%> (ø)
...ry-resource-detector-github/src/detectors/index.ts	100.00% <100.00%> (ø)
...ector-github/test/detectors/GitHubDetector.test.ts	100.00% <100.00%> (ø)