Add autodetect arg to external table creation in GCS-to-BQ operator

[shahkshitij15]

Added autodetect in external table creation for GCS to BQ operator
Now one can create the external table without passing any schema

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
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[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

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[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.

[potiuk]

Tests are failing @shahkshitij15

[shahkshitij15]

Hey! I have updated the test cases. Now it should work @potiuk
Please approve @turbaszek

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[shahkshitij15]

Hey! With reference to issue #22063, how shall I test the proper working of my code? @potiuk

[potiuk]

Here you can find some helpful guidelines: https://github.com/apache/airflow/blob/main/dev/README_RELEASE_PROVIDER_PACKAGES.md#verify-by-contributors

[potiuk]

Improved instructions added in PR #22087