feat(spanner): equality for copyable classes

[dbolduc]

Part of the work for #2310

The classes are:

• spanner::CreateInstanceRequestBuilder
• spanner::UpdateInstanceRequestBuilder

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 76c6d91ff52969ef00b310c97f11ec7768685f0e

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

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

[codecov]

Codecov Report

Merging #9648 (76c6d91) into main (a508bb6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #9648      +/-   ##
==========================================
- Coverage   94.35%   94.35%   -0.01%     
==========================================
  Files        1493     1493              
  Lines      138786   138813      +27     
==========================================
+ Hits       130956   130980      +24     
- Misses       7830     7833       +3     

Impacted Files	Coverage Δ
...le/cloud/spanner/create_instance_request_builder.h	100.00% <100.00%> (ø)
...ud/spanner/create_instance_request_builder_test.cc	100.00% <100.00%> (ø)
...le/cloud/spanner/update_instance_request_builder.h	100.00% <100.00%> (ø)
...ud/spanner/update_instance_request_builder_test.cc	100.00% <100.00%> (ø)
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	84.21% <0.00%> (-0.66%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.75% <0.00%> (-0.57%)	⬇️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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