fix(storage): disable XML with custom endpoints

[coryan]

The library automatically uses XML for some operations (for performance
reasons). This does not work well when using a custom endpoint, as the
endpoints for XML are not set. We completely disable XML if the
application sets a custom endpoint, as in this case they (most likely)
want to use an emulator.

Fixes #5072

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This change is 

[codecov]

Codecov Report

Merging #5074 into master will increase coverage by 0.00%.
The diff coverage is 95.00%.

@@           Coverage Diff           @@
##           master    #5074   +/-   ##
=======================================
  Coverage   94.03%   94.03%           
=======================================
  Files         983      983           
  Lines       76480    76558   +78     
=======================================
+ Hits        71917    71994   +77     
- Misses       4563     4564    +1     

Impacted Files	Coverage Δ
google/cloud/storage/client_options.h	100.00% <ø> (ø)
...torage/tests/object_basic_crud_integration_test.cc	86.17% <91.66%> (+5.73%)	⬆️
google/cloud/storage/client_options.cc	96.55% <100.00%> (+0.06%)	⬆️
google/cloud/storage/internal/curl_client.cc	97.29% <100.00%> (+0.01%)	⬆️
google/cloud/storage/internal/curl_client.h	100.00% <100.00%> (ø)
google/cloud/storage/internal/curl_client_test.cc	99.44% <100.00%> (+0.02%)	⬆️
google/cloud/grpc_error_delegate.cc	95.83% <0.00%> (-4.17%)	⬇️
google/cloud/storage/internal/retry_client.cc	98.43% <0.00%> (-0.23%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.45% <0.00%> (+0.10%)	⬆️
google/cloud/storage/parallel_upload.cc	99.61% <0.00%> (+0.38%)	⬆️
... and 1 more

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