feat: skip generating grpc-* directory if transport is rest

[JoeWang1127]

In this PR:

• Fix The generated google compute library has grpc-* directory when transport is rest #1981
• Fix the issue that the transport can only be parsed as grpc+rest or grpc.
• Refactor utility functions in parsing transport, rest_numeric_enums and include_samples and add unit tests.

[JoeWang1127]

Test locally with google compute, a rest only library.

Will add more integration tests in another PR.

[meltsufin]

@JoeWang1127 There seems to be a lot more going on here than just "skip generating grpc-* directory if transport is rest". Can you explain more what you're trying to do in this PR or split it into multiple? It will be easier to review.

[JoeWang1127]

@JoeWang1127 There seems to be a lot more going on here than just "skip generating grpc-* directory if transport is rest". Can you explain more what you're trying to do in this PR or split it into multiple? It will be easier to review.

I added a PR description. The refactor happens here because the utilitiy functions that parsing transport, rest_numeric_enums and include_samples are similar.

[sonarqubecloud]

[gapic-generator-java-root] Kudos, SonarCloud Quality Gate passed!   

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[sonarqubecloud]

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[sonarqubecloud]

[java_showcase_unit_tests] Kudos, SonarCloud Quality Gate passed!   

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