doc(bigtable): add *Client samples

[coryan]

Add examples showing how to override the default endpoint and default authentication.

Fixes #10143 and fixes #10144

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 5bdc02c53c805b20a59c9a2499bde19c5c374156

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[codecov]

Codecov Report

Base: 93.98% // Head: 93.98% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (593432e) compared to base (7ddabb9).
Patch coverage: 95.55% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10149      +/-   ##
==========================================
- Coverage   93.98%   93.98%   -0.01%     
==========================================
  Files        1524     1525       +1     
  Lines      140472   140517      +45     
==========================================
+ Hits       132024   132061      +37     
- Misses       8448     8456       +8     

Impacted Files	Coverage Δ
google/cloud/bigtable/examples/client_samples.cc	95.55% <95.55%> (ø)
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.51%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️
google/cloud/internal/async_rpc_details.h	100.00% <0.00%> (+5.00%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 593432e1b69e9cef89df7931d15919219163daab

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

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

[dbolduc]

Thanks for the samples!