Add union to test_all_types, and arrow and json R/W

[Mause]

This started out adding support to the Python .description field for the union data type, but I realised that we didn't have it in the test_all_types table function.

• Added support for R/W in Arrow, specifically as a Sparse Union. Reading Dense Unions will still fail
• Added read support for Unions in JSON. Write support was already there, so I just followed the existing pattern
• Corrected a number of locations where the Union tag type was incorrect - a uint8_t instead of a int8_t
• Parquet doesn't support unions (see https://issues.apache.org/jira/browse/PARQUET-756)
• Added support for reading unions into numpy/pandas (as object type)
• R support was left on the table, I'm not familiar enough with R to do this without guidance
• Basic Java union read support is there, but only stringly typed - I'll add full support when I added Struct support (soon)

[lnkuiper]

Thanks for the PR! I have left some comments about the JSONTransform implementation.

I'm also wondering about how other systems map from JSON to UNION. Do you have any idea? Perhaps we're the only system out there that does this. I always thought that reading JSON like this:

{"value": 42}
{"value": [1, 2, 3]}

Could be read using UNION(int INTEGER, int_list INT[]) without having to wrap them in an object with the keys "int" or "int_list".

But now that I see your implementation, I'm not sure what makes the most sense, especially since I'm not a heavy JSON user myself.

[Mytherin]

Thanks for the PR! Looks great.

As this is adding unions to the arrow conversion - could we add some union-specific arrow conversion tests in the Python client? In particular nested unions, unions with lists/structs inside them, unions inside lists/structs, all are not covered yet but could potentially cause problems.

[Mause]

Failures don't look related my changes?

[Mytherin]

Could you merge with master?

[Maxxen]

👏👏👏