Add udf / udaf decorators

[timsaucer]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Based on this recommendation apache/datafusion-site#17 (comment) we can add decorators for UDFs to make the even easier to write.

Describe the solution you'd like
Add decorators for udf and udaf

Describe alternatives you've considered

These aren't required but they will make it even more pythonic for users.

[crystalxyz]

Hi @timsaucer , I have just attempted to implement the decorators. Would you mind reviewing it and providing some comments for me? Since it involves an API change, I would like to confirm with you before adding documentation :)