Add SQL function array_frequency

[jzhou1318]

Test plan - Wrote unit tests to check the function works with varchar and bigints. Can deal with duplicates.

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== RELEASE NOTES ==

General Changes
* Add function :func:`array_frequency'

[linux-foundation-easycla]

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• ✅ Jennifer Zhou (438697a2cf5980bc4972518c29718f1c2991ad8c, f0ef293691c117c5122bcf336874a7cca3bf0de0, 4929161b18d93a5873551ada5e7702f17bf25ee6, 7221180fa78d62c665c9b2e21af396075cc713c0)

[prithvip]

LGTM, other than adding tests and a style nit.

[prithvip]

I think we should also add tests for the empty array case, and for nulls in the array.

[rongrong]

Let's also add test for empty array as @prithvip suggested. array_frequency(cast(array[] as array(bigint). Also, for bigint / varchar, either use all upper case or all lower case. SQL is case in sensitive, I think here you can use all lower case.

[rongrong]

Hmm, while this behavior is sort of reasonable, I wonder whether this is expected. @kaikalur what do you think should be the right outcome if the input array only has null elements?

[kaikalur]

This seems reasonable (especially because we can't represent null as a key in the result map)

[jzhou1318]

I changed the code to ignore null elements (can deal with arrays that have both null and non-null elements), so array of nulls now returns an empty map.

[rongrong]

We also need to update the array.rst file to add documentation for this function.

[rongrong]

For release note, it should end with backquote rather than double quote.

[rongrong]

again, lowercase bigint

[rongrong]

lower case varchar

[rongrong]

lowercase map(bigint, int)

[rongrong]

lowercase map(varchar, int)

Generally we use () rather than <> in type signature.

[rongrong]

Looks good! One final nit: Please change the commit title to Add SQL function array_frequency (We normally use the verb "add" when adding new features, also upper case first letter)

[kaikalur]

LGTM