Add autodetect arg in BQCreateExternalTable Operator

[hankehly]

Closes #22129

This PR adds an autodetect flag (with a backward compatible default value) to the BigQueryCreateExternalTableOperator constructor. If schema_fields and schema_object are blank, and the autodetect flag is set to True, BigQuery will attempt to auto-detect the input data schema.

Todo:

• update code
• update documentation
• update unit tests
• check manually

Documentation updates

Added the following line to the Google provider package docs.

Manual checks

I uploaded a CSV file containing the following to a Google Cloud Storage bucket in the us-east1 region.

name,age
Bill,42
Sally,15

I then ran a test DAG to create an external table based on the above CSV file. I enable BigQuery schema auto-detection by setting the autodetect flag to True.

BigQueryCreateExternalTableOperator(
    task_id="test",
    bucket="***",
    source_objects=["test.csv"],
    destination_project_dataset_table = "***",
    source_format = "CSV",
    autodetect = True,
    skip_leading_rows=1,
    location = "us-east1",
)

The table is created with the correct field names and types. The table configuration also shows that schema auto-detection was enabled.

[boring-cyborg]

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[hankehly]

@turbaszek @mik-laj Please review this PR at your earliest convenience.

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!