feat(resource-util): add App Engine detector

[aabmass]

e2e tests passing in this PR

[codecov]

Codecov Report

Merging #510 (8e05171) into main (9593cb9) will increase coverage by 0.12%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
+ Coverage   95.84%   95.97%   +0.12%     
==========================================
  Files          17       18       +1     
  Lines         602      646      +44     
  Branches      113      117       +4     
==========================================
+ Hits          577      620      +43     
- Misses         25       26       +1     

Impacted Files	Coverage Δ
...es/opentelemetry-resource-util/src/detector/gae.ts	96.66% <96.66%> (ø)
...entelemetry-resource-util/src/detector/detector.ts	100.00% <100.00%> (ø)
packages/opentelemetry-resource-util/src/index.ts	100.00% <100.00%> (ø)

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[dashpole]

Nice!