implement gcs_schema_object for BigQueryCreateExternalTableOperator

[attilaszombati]

Before this PR:

The BigQueryCreateExternalTableOperator can work with the schema object if the JSON file is in the same bucket as the data source objects.

After this PR:

The developers have the ability to define a different bucket where the schema object is located like the implementation of the BigQueryCreateEmptyTableOperator.

[boring-cyborg]

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[pankajastro]

shall we add a test for it or update some existing tests?

[attilaszombati]

@pankajastro good point, I will add some tests to cover this parameter

[attilaszombati]

@pankajastro I found one test for this operator, doesn't seem a good idea to create a new test case just for this parameter. If you think it's necessary then I can create a separate one.

For now, I just added the new parameter to the test as well. What do you think?

[boring-cyborg]

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