feat: better support for PSC and VPC-SC

[coryan]

When using PSC (Private Service Connect) or VPC-SC (Virtual Private
Cloud - Service Controls) we need to specify the authority in each
gRPC request. This is the HTTP/2 analog of a Host header, and allows
applications to access a service while using a custom endpoint. For
example, the target endpoint could be pubsub-custom.p.googleapis.com,
but the authority field remains pubsub.googleapis.com.

Part of the work for #3317

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[codecov]

Codecov Report

Merging #8453 (404f811) into main (291e9dd) will decrease coverage by 0.00%.
The diff coverage is 54.83%.

@@            Coverage Diff             @@
##             main    #8453      +/-   ##
==========================================
- Coverage   93.85%   93.85%   -0.01%     
==========================================
  Files        1444     1444              
  Lines      124189   124223      +34     
==========================================
+ Hits       116562   116587      +25     
- Misses       7627     7636       +9     

Impacted Files	Coverage Δ
generator/internal/metadata_decorator_generator.cc	59.37% <ø> (ø)
...ud/pubsublite/internal/admin_metadata_decorator.cc	0.00% <0.00%> (ø)
...d/pubsublite/internal/cursor_metadata_decorator.cc	0.00% <0.00%> (ø)
...nternal/partition_assignment_metadata_decorator.cc	0.00% <0.00%> (ø)
...ubsublite/internal/publisher_metadata_decorator.cc	0.00% <0.00%> (ø)
...bsublite/internal/subscriber_metadata_decorator.cc	0.00% <0.00%> (ø)
...sublite/internal/topic_stats_metadata_decorator.cc	0.00% <0.00%> (ø)
...internal/golden_kitchen_sink_metadata_decorator.cc	84.09% <50.00%> (-1.63%)	⬇️
.../internal/golden_thing_admin_metadata_decorator.cc	96.38% <50.00%> (-1.15%)	⬇️
google/cloud/internal/populate_common_options.cc	100.00% <100.00%> (ø)
... and 7 more

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[dbolduc]

Thanks for figuring this out. The changes LGTM.

I was confused by the scope of the PR (generated metadata decorators + spanner) but I see that you have a handle on the remaining work in the issue description.