cleanup: const-correctness for unified credentials

[coryan]

When mapping google::cloud::Credentials to the implementation (gRPC or REST) we don't need a std::shared_ptr<> or (shudder) std::shared_ptr<> const&. The visitor can work with a const&.

Motivated by #12064 (comment)

---

This change is 

[codecov]

Codecov Report

Patch coverage: 94.91% and project coverage change: -0.01 ⚠️

Comparison is base (57acdc1) 93.68% compared to head (e8dcee7) 93.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12073      +/-   ##
==========================================
- Coverage   93.68%   93.68%   -0.01%     
==========================================
  Files        2012     2012              
  Lines      175425   175425              
==========================================
- Hits       164347   164342       -5     
- Misses      11078    11083       +5     

Impacted Files	Coverage Δ
google/cloud/internal/unified_grpc_credentials.h	100.00% <ø> (ø)
...cloud/storage/internal/unified_rest_credentials.cc	78.57% <85.71%> (ø)
google/cloud/internal/unified_grpc_credentials.cc	82.97% <87.50%> (ø)
google/cloud/internal/unified_rest_credentials.cc	94.91% <87.50%> (-1.70%)	⬇️
google/cloud/internal/credentials_impl.cc	100.00% <100.00%> (ø)
google/cloud/internal/credentials_impl.h	100.00% <100.00%> (ø)
google/cloud/internal/credentials_impl_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/curl_rest_client.cc	96.35% <100.00%> (ø)
...cloud/internal/grpc_impersonate_service_account.cc	100.00% <100.00%> (ø)
.../oauth2_impersonate_service_account_credentials.cc	100.00% <100.00%> (ø)
... and 5 more

... and 4 files with indirect coverage changes

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