cleanup(GCS+gRPC): move to storage_internal

[coryan]

Specifically GrpcClient.

Part of the work for #8929

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This change is 

[google-cloud-cpp-bot]

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For commit: 3701e9294794c56085933515dda38d83d22de1b4

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[codecov]

Codecov Report

Base: 94.23% // Head: 94.23% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3701e92) compared to base (d4f61ad).
Patch coverage: 98.66% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10015   +/-   ##
=======================================
  Coverage   94.23%   94.23%           
=======================================
  Files        1522     1522           
  Lines      141361   141379   +18     
=======================================
+ Hits       133212   133230   +18     
  Misses       8149     8149           

Impacted Files	Coverage Δ
google/cloud/storage/grpc_plugin_test.cc	100.00% <ø> (ø)
google/cloud/storage/internal/grpc_client.h	100.00% <ø> (ø)
...e/internal/grpc_client_insert_object_media_test.cc	100.00% <ø> (ø)
...d/storage/internal/grpc_client_read_object_test.cc	96.77% <ø> (ø)
.../storage/internal/grpc_client_upload_chunk_test.cc	100.00% <ø> (ø)
google/cloud/storage/internal/grpc_client.cc	98.81% <97.29%> (+<0.01%)	⬆️
google/cloud/storage/async_client.cc	100.00% <100.00%> (ø)
google/cloud/storage/grpc_plugin.cc	100.00% <100.00%> (ø)
...le/cloud/storage/internal/async_connection_impl.cc	91.66% <100.00%> (ø)
...oud/storage/internal/async_connection_impl_test.cc	100.00% <100.00%> (ø)
... and 11 more

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[scotthart]

Reviewed 15 of 15 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)