feat(clients): modular endpoints resolution

[trivikr]

Issue

Internal JS-2773
Follow-up to #2701

Description

Makes endpoint resolution modular as follows:

• Removes immutable variables.
• Removes switch statements.
• Calls regionInfo utility functions from @aws-sdk/config-resolver, and passes appropriate options.

Testing

Integration tests are successful:

$ 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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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@7c6a790). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2704   +/-   ##
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  Coverage        ?   61.30%           
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  Files           ?      535           
  Lines           ?    27442           
  Branches        ?     6698           
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  Hits            ?    16823           
  Misses          ?    10619           
  Partials        ?        0           

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[AllanZhengYP]

Approve this as a follow up to #2696

[github-actions]

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