add PARTITION BY option for CopyInto

[azban]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes #430

2. Fill out the following pre-review checklist:

   • [x] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding new credentials
   • [ ] I am adding a new dependency

3. Please describe how your code solves the related issue.

This adds an argument to CopyInto which can be used to specify a string value for PARTITION BY. I have a few questions about implementation:

1. Would it be better to adhere to the convention of having functions that do some validation similar to those used for CopyInto.copy_options? If so, is there any particular validation to make sense?
2. Is there a preference towards implementing this as a clause rather than allowing for a string? I'm not very familiar with custom clauses, and I don't know of a similar implementation that would be useful to base this on.
3. Unrelated to implementation, but is there a straightforward way to run tests locally? i.e. is there an example parameters file that can be used?

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[azban]

I have read the CLA Document and I hereby sign the CLA

[azban]

recheck

[azban]

can anyone please take a look at this?

[azban]

what's the process for this repo considering and accepting PR's? should we assume we should maintain our own forks long-term?