feat(bigtable): generate admin APIs

[dbolduc]

Generate the Bigtable Admin APIs again, which were reverted in #7667.

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This change is 

[google-cloud-cpp-bot]

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For commit: 8da41e21721122b437fb9e7c545d4d776d2aa378

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[codecov]

Codecov Report

Merging #7700 (8da41e2) into main (6557aba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7700   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files        1254     1254           
  Lines      113589   113590    +1     
=======================================
+ Hits       108241   108245    +4     
+ Misses       5348     5345    -3     

Impacted Files	Coverage Δ
...le/cloud/bigtable/testing/table_integration_test.h	75.00% <ø> (ø)
...e/cloud/bigtable/testing/table_integration_test.cc	80.00% <100.00%> (+0.11%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	86.18% <0.00%> (+0.65%)	⬆️

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