feat: Add callable getters for non-eligible or non-enabled REST methods

[lqiu96]

This is a PR for Part 1 of #1117 (Override method with clearer exception messages for non-eligible and non-enabled Service RPCs). Opening this while I look at other possible approaches:

Other approaches looked at/ to revisit:

• Override createCallableClassMembers to return Map<String, Expr> (Return either VariableExpr or ThrowExpr)
• Create a duplicate createCallableClassMembers (i.e. createInvalidCallableClassMembers) that copies functionality of createCallableClassMembers but returns Map<String, ThrowExpr>

Both approaches above had an issue when setting the return type for the callableGetter. ThrowExpr's type is always set as UnsupportedOperationException but the MethodDefinition's return type is not (i.e. for Streams it would be ServerStreamCallable or ClientStreamCallable/ Unary is UnaryCallable vs. Operation, etc.). Would need potentially need another mapping between callableName and method return type for ThrowExprs.

This PR's implementation is copied from: https://github.com/googleapis/gapic-generator-java/blob/8c5e17ba325b7711f9ba9501992ab48e736ffc18/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubClassComposer.java#L284-L302

• Use getCallableType(protoMethod) to get the correct return type for the RPC
• ThrowExpr's return type is set to UnsupportedOperationException

[lqiu96]

@blakeli0 Can you take another look whenever you get a chance?

[lqiu96]

PR comments resolved. Updated message to Not implemented: {callableName}(). REST transport is not implemented for this method yet.. Adding automerge for this PR

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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