feat(clients): allow setting path prefix from custom endpoints

[AllanZhengYP]

Issue

Resolves: #1399
Related to: smithy-lang/smithy-typescript#406

Description

Allow the path in custom endpoints to be the prefix of RPC protocols.

Testing

Integration test

yarn test:integration-legacy                            
yarn run v1.22.10
$ cucumber-js --fail-fast
...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

150 scenarios (150 passed)
523 steps (523 passed)
1m37.017s
Done in 102.21s.

Additional context

Add any other context about the PR here.

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Codecov Report

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The diff coverage is n/a.

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  Hits            ?    16834           
  Misses          ?    10619           
  Partials        ?        0           

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