[jrmccluskey]

Migrates Python BigQuery IO usage to the modern client, with the legacy apitools-generated client available via passing --experiments=use_legacy_bigquery_client or --experiments=use_legacy_bq_client. Also adds documentation on the compatibility shims introduced in #39998 (but these are unlikely to be used.)

Part of #37261

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[codecov]

Codecov Report

❌ Patch coverage is 48.89317% with 531 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.12%. Comparing base (22a237b) to head (98d1909).
⚠️ Report is 123 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/io/gcp/bigquery_tools.py	48.77%	440 Missing ⚠️
...thon/apache_beam/io/gcp/bigquery_change_history.py	26.19%	31 Missing ⚠️
sdks/python/apache_beam/io/gcp/bigquery.py	44.44%	30 Missing ⚠️
...python/apache_beam/io/gcp/bigquery_schema_tools.py	70.00%	12 Missing ⚠️
...s/python/apache_beam/io/gcp/bigquery_file_loads.py	47.36%	10 Missing ⚠️
...ython/apache_beam/io/gcp/bigquery_read_internal.py	58.33%	5 Missing ⚠️
...ython/apache_beam/io/gcp/tests/bigquery_matcher.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40340      +/-   ##
============================================
- Coverage     57.25%   56.12%   -1.13%     
+ Complexity     3695     2290    -1405     
============================================
  Files          1200     1122      -78     
  Lines        194194   178833   -15361     
  Branches       3889     1486    -2403     
============================================
- Hits         111183   100377   -10806     
+ Misses        79406    75935    -3471     
+ Partials       3605     2521    -1084     

Flag	Coverage Δ
python	79.30% <48.89%> (-0.53%)	⬇️

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