feat: generate client, transport, and unit test templates for resumable uploads

[parthea]

• Add client, transport, and unit test template support for resumable upload RPCs.

• Resumable upload methods accept an optional ResumableUploadConfig on the client and return ResumableUploadSession / AsyncResumableUploadSession from google.api_core.resumable_transfer. Since resumable uploads use HTTP, the gRPC and gRPC-async transports delegate resumable upload calls to the corresponding REST transport.

[gemini-code-assist]

Code Review

This pull request adds support for resumable upload methods in the GAPIC generator, wrapping responses in ResumableUploadSession or AsyncResumableUploadSession and ensuring these methods route over REST even when gRPC transport is initialized. It updates templates, transports, and unit tests to support this protocol. The reviewer identified a critical issue in grpc_asyncio.py.j2 where AsyncRestTransport is instantiated on every call, which is highly inefficient and causes resource leaks; they suggested caching the transport instance instead.

[daniel-sanche]

It seems like there could be some runtime issues here. Can you make sure we have full coverage?

[snippet-bot]

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[daniel-sanche]

I wanted to bring all my remaining comments into one place, although I know some of these are already being addressed

[daniel-sanche]

LGTM