cleanup(GCS+gRPC): move to storage_internal

[coryan]

Specifically HybridClient.

Part of the work for #8929

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 89e1cee722e41f268a0f03933be2e9031eeb1744

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• Kokoro Logs (requires auth)

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

[codecov]

Codecov Report

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

Coverage data is based on head (89e1cee) compared to base (6580ede).
Patch coverage: 95.08% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10012   +/-   ##
=======================================
  Coverage   94.23%   94.23%           
=======================================
  Files        1522     1522           
  Lines      141370   141373    +3     
=======================================
+ Hits       133214   133223    +9     
+ Misses       8156     8150    -6     

Impacted Files	Coverage Δ
google/cloud/storage/grpc_plugin_test.cc	100.00% <ø> (ø)
google/cloud/storage/internal/hybrid_client.h	100.00% <ø> (ø)
google/cloud/storage/internal/hybrid_client.cc	94.06% <94.82%> (+0.10%)	⬆️
google/cloud/storage/grpc_plugin.cc	100.00% <100.00%> (ø)
...loud/storage/internal/grpc_client_failures_test.cc	100.00% <100.00%> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/storage/parallel_upload.cc	98.61% <0.00%> (+0.34%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.65% <0.00%> (+0.50%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	98.31% <0.00%> (+0.56%)	⬆️
... and 2 more

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