cleanup(bigtable): store project id in InstanceAdmin

[dbolduc]

Part of the work for #7534

We need to store the project id ourselves, so we can eventually drop our dependence on the legacy client_.

The full change (short of a CHANGELOG entry) is here: https://github.com/dbolduc/google-cloud-cpp/compare/bigtable-convert-instance-admin

The diff is 4500 lines. I am trying to break it up into something more palatable. My plan is to break it into the following PRs:

1. project id change (this PR)
2. add the logic for storing a generated Connection (without ever using the new connection)
3. add the mechanism to convert bigtable policies passed to the constructor into Options.
4. (BREAKING): Reimplement InstanceAdmin in terms of the generated Connection. Rewrite the unit tests. Update the CHANGELOG. (This will be a tedious PR).
5. Gut the InstanceAdminClient class. Delete unused classes and their tests.

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This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

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For commit: 43a035f4cdb98ff899b1c2c040d5cd26c2fac236

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[codecov]

Codecov Report

Merging #8051 (43a035f) into main (ebc8f09) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8051      +/-   ##
==========================================
- Coverage   95.11%   95.10%   -0.01%     
==========================================
  Files        1313     1313              
  Lines      117659   117662       +3     
==========================================
- Hits       111907   111905       -2     
- Misses       5752     5757       +5     

Impacted Files	Coverage Δ
google/cloud/bigtable/instance_admin.h	100.00% <100.00%> (ø)
...e/cloud/spanner/testing/cleanup_stale_instances.cc	61.29% <0.00%> (-19.36%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	85.52% <0.00%> (-0.66%)	⬇️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.37% <0.00%> (-0.51%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️
google/cloud/bigtable/internal/common_client.cc	97.14% <0.00%> (+1.42%)	⬆️

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