Fix behavior for reattach_state parameter in BigQueryInsertJobOperator

[VladaZakharova]

When trying to attach to RUNNING (but also any other state too) BQ Job using BigQueryInsertJobOperator, even though reattach_states={'PENDING', 'RUNNING'} parameter is set and Job is still running, ValueError: Job already begun is thrown.

Attaching to DONE job, which according to documentation shouldn't be possible (from airflow doc: if job with given id already exists then it tries to reattach to the job if its not done and its state is in reattach_states. If the job is done the operator will raise AirflowException.), but throws an error, as well as with any other Job type because of the validation in job._begin() call and fails if it's different than None.

This PR fixes this by adding checks of the current state before going to execute the Job in deferrable or synch mode.

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[VladaZakharova]

Hi @potiuk @Lee-W ! Can you please check changes in this PR? Thanks!