feat(clients): make endpoints resolution modular

[trivikr]

Issue

Internal JS-2773
Refs for PoC PR: aws#2696

This PR will be posted upstream, after aws#2701 is merged

Description

Makes endpoint resolution modular as follows:

• Removes immutable variables.
• Removes switch statements.
• Calls region info computation logic to config-resolver module added in feat(config-resolver): add getRegionInfo helper functions aws/aws-sdk-js-v3#2701.
• Calls region info utility function, and passes appropriate options.

Testing

Integration tests are succesful:

$ yarn test:integration
 PASS  clients/client-transcribe-streaming/test/index.integ.spec.ts (31.631 s)
  TranscribeStream client
    ✓ should stream the transcript (28799 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        32.11 s
Ran all test suites.
Done in 32.76s.

$ yarn test:integration-legacy 
yarn run v1.22.11
$ cucumber-js --fail-fast
...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

150 scenarios (150 passed)
523 steps (523 passed)
1m35.810s
Done in 99.40s.

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[trivikr]

PR posted upstream at aws#2704