feat(gax): introduce GrpcRustClient skeleton

[joshuatants]

For #5991.

Introducing a skeleton unblocks working on the changes needed for Storage for both the hand-written (#6000) and GAPIC (changes to be made to the relevant templates in librarian) clients.

[gemini-code-assist]

Code Review

This pull request introduces a new, conditionally compiled grpc_rust module containing stubs for GrpcRustClient and GrpcRustStreaming under the google_cloud_unstable_grpc_rust configuration. The feedback suggests renaming the getter methods get_polling_error_policy and get_polling_backoff_policy to omit the get_ prefix, aligning with Rust naming conventions and the repository style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (affbeb2) to head (3f0287f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5999   +/-   ##
=======================================
  Coverage   97.73%   97.73%           
=======================================
  Files         244      244           
  Lines       61096    61096           
=======================================
  Hits        59710    59710           
  Misses       1386     1386           

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[coryan]

I think you want to add this to:

google-cloud-rust/.gcb/builds/triggers/main.tf

Lines 37 to 42 in 3774eba

	unstable_flags = join(" ", [
	"--cfg google_cloud_unstable_trust_boundaries",
	"--cfg google_cloud_unstable_storage_bidi",
	"--cfg google_cloud_unstable_grpc_server_streaming",
	"--cfg google_cloud_unstable_tracing",
	])

So the new flag is tested.