samples(bigtable): build admin samples with cmake

[dbolduc]

These samples were never built with cmake. We run them with bazel in the integration-production build.

We still do not run them, but at least they are built in clang-tidy.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: f91979958a4efd11a8ceaee40aaa41fa144cc3e3

• 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

Base: 93.84% // Head: 93.84% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (f919799) compared to base (ba4669e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10246      +/-   ##
==========================================
- Coverage   93.84%   93.84%   -0.01%     
==========================================
  Files        1541     1541              
  Lines      142163   142163              
==========================================
- Hits       133418   133417       -1     
- Misses       8745     8746       +1     

Impacted Files	Coverage Δ
google/cloud/pubsub/subscriber_connection_test.cc	97.81% <0.00%> (-0.55%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.34%)	⬇️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️

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