cleanup(GCS+gRPC): remove struct GrpcOwnerParser

[coryan]

The struct was just serving as a namespace. This also starts the move of the GCS+gRPC implementation details to the storage_internal namespace.

Part of the work for #8929

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: a1aac9768a11e0ed366db82488bac41aff8d5643

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 94.24% // Head: 94.24% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (a1aac97) compared to base (0777ded).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9837      +/-   ##
==========================================
- Coverage   94.24%   94.24%   -0.01%     
==========================================
  Files        1495     1495              
  Lines      139834   139834              
==========================================
- Hits       131783   131780       -3     
- Misses       8051     8054       +3     

Impacted Files	Coverage Δ
...ud/storage/internal/grpc_bucket_metadata_parser.cc	92.30% <100.00%> (ø)
...ud/storage/internal/grpc_object_metadata_parser.cc	97.19% <100.00%> (ø)
google/cloud/storage/internal/grpc_owner_parser.cc	83.33% <100.00%> (ø)
...e/cloud/storage/internal/grpc_owner_parser_test.cc	100.00% <100.00%> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.