feat: remove DataClient and it's usages

[mpeddada1]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @mpeddada1, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request undertakes a significant refactoring effort within the Bigtable C++ client library. The primary goal is to eliminate the deprecated DataClient interface and its legacy implementations, consolidating all data interaction paths through the DataConnection interface. This change simplifies the client's internal structure, removes outdated code, and aligns the library with current best practices for API interaction.

Highlights

• DataClient Removal: The deprecated DataClient interface and all its associated implementations, including inprocess_data_client and logging_data_client, have been completely removed from the library.
• Transition to DataConnection: All data operations within the Table class now exclusively use the DataConnection interface, streamlining the client's architecture and ensuring consistency with modern design patterns.
• Legacy Code Cleanup: Several legacy asynchronous and synchronous data client implementations, such as legacy_async_bulk_apply, legacy_async_row_reader, legacy_async_row_sampler, and legacy_row_reader, along with their respective tests, have been removed.
• Build System Updates: The CMakeLists.txt and bigtable_client_testing.bzl files have been updated to remove references to the deleted DataClient related files and tests.
• Example and Integration Test Refactoring: All examples and integration tests have been modified to reflect the removal of DataClient and now utilize the DataConnection interface for Bigtable operations.

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[gemini-code-assist]

Code Review

This pull request is a large but important refactoring to remove the deprecated DataClient and all its usages. The changes are extensive, touching build files, examples, tests, and the core Table implementation. The removal of the legacy code paths simplifies the Table class significantly, making it delegate all operations to the DataConnection. The tests have been updated to reflect this change, removing the now-obsolete parameterization that tested both DataClient and DataConnection paths. Overall, this is an excellent cleanup that improves the maintainability of the codebase. The changes look correct and complete.

[codecov]

Codecov Report

❌ Patch coverage is 99.51691% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.87%. Comparing base (600d394) to head (0378315).

Files with missing lines	Patch %	Lines
...ogle/cloud/bigtable/tests/data_integration_test.cc	97.77%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15847      +/-   ##
======================================================
- Coverage               92.94%   92.87%   -0.08%     
======================================================
  Files                    2454     2422      -32     
  Lines                  227401   223243    -4158     
======================================================
- Hits                   211368   207342    -4026     
+ Misses                  16033    15901     -132     

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