fix issues with regional endpoints and add integ tests

[AllanZhengYP]

• fix a bug in regional endpoint (STS fails with AWS_STS_REGIONAL_ENDPOINTS = regional #2991)
• add integ tests testing actual regional endpoint calls
• add client's configuration type definitions for s3UsEast1RegionalEndpoint and s3UseArnRegion

Checklist

• npm run test passes
• .d.ts file is updated
• changelog is added, npm run add-change
• run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed
• run npm run integration if integration test is changed

[codecov-io]

Codecov Report

Merging #2994 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2994      +/-   ##
==========================================
- Coverage   97.04%   97.04%   -0.01%     
==========================================
  Files         326      326              
  Lines        9477     9473       -4     
  Branches     1743     1743              
==========================================
- Hits         9197     9193       -4     
  Misses        280      280

Impacted Files	Coverage Δ
lib/services/s3.js	98.19% <100%> (-0.01%)	⬇️
lib/services/sts.js	100% <100%> (ø)	⬆️

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