Move BigQuery Xlang tests to apache_beam/io/external/

[ahmedabu98]

BigQuery xlang tests are currently in bigquery_write_it_test, so they get picked up by Python Postcommits and fail because the right environment isn't set up.

Fixes #26016

[ahmedabu98]

Run Python_Xlang_Gcp_Direct PostCommit

[ahmedabu98]

Run Python_Xlang_Gcp_Dataflow PostCommit

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[ahmedabu98]

R: @Abacn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[Abacn]

Thanks for the fix!

[Abacn]

Xlang test suites passed:
https://ci-beam.apache.org/job/beam_PostCommit_Python_Xlang_Gcp_Direct_PR/37/testReport/apache_beam.io.external.xlang_bigqueryio_it_test/BigQueryXlangStorageWriteIT/

https://ci-beam.apache.org/job/beam_PostCommit_Python_Xlang_Gcp_Dataflow_PR/28/testReport/apache_beam.io.external.xlang_bigqueryio_it_test/BigQueryXlangStorageWriteIT/

beam_PreCommit_Python_Coverage_Commit fails apache_beam.runners.portability.portable_runner_test.PortableRunnerTestWithSubprocesses.test_error_message_includes_stage unrelated, other pending jobs are succeeded not updating to GitHub UI. Merging for now