Add template field to S3ToRedshiftOperator

[alextgu]

Add template field for method for S3ToRedshiftOperator

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[boring-cyborg]

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[josh-fell]

@alextgu Can you take a look at the failing tests when you get a chance?

[o-nikolas]

I patched the tests so that the build is green. @vincbeck or @josh-fell can you have a very quick look?

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.