fix(cmake)!: only compile protos if asked - storage

[dbolduc]

Part of the work for #8022

This one is a bit weird. I think the protos should only be built with GCS+gRPC. (As opposed to, storage IN LISTS GOOGLE_CLOUD_CPP_ENABLE AND GOOGLE_CLOUD_CPP_ENABLE_GRPC)

Conditionally include the raw gRPC client in the throughput benchmark only if GCS+gRPC.

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This change is 

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (d000cf8) 93.62% compared to head (956be77) 93.62%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12481      +/-   ##
==========================================
- Coverage   93.62%   93.62%   -0.01%     
==========================================
  Files        2039     2039              
  Lines      180676   180677       +1     
==========================================
  Hits       169151   169151              
- Misses      11525    11526       +1     

Files Changed	Coverage Δ
.../cloud/storage/benchmarks/throughput_experiment.cc	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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