surfer: Support complex mappings between API methods and gcloud commands

[quirogas]

The current generator assumes a straightforward mapping between a single API method and a gcloud command verb. However, gcloud conventions sometimes require multiple methods to map to a single command.

Context & Rationale

• https://github.com/googleapis/librarian/pull/3294/changes#r2644174242

Implementation Considerations

• Partial Management: Ensure that _partials files for different tracks are correctly managed when multiple methods are involved.

Relevant files

• internal/surfer/gcloud/generate.go
• internal/surfer/gcloud/utils/method.go