Add IPC support for Union Array

[paddyhoran]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Although UnionArray is implemented it has no IPC support.

Describe the solution you'd like
UnionArray should pass the integration tests.

Describe alternatives you've considered
NA

Additional context
NA

[sum12]

If I am reading the code correctly arrow/src/ipc/reader::create_array needs to know how to recognize a UnionArray and create one accordingly. I can imagine using arrow/src/array/array_union.new or arrow/src/array/array_union::try_new.

if this is still relevant. Could you help me break this down to in some steps ? I would like to contribute this.

[paddyhoran]

Hi @sum12,

I think it would make more sense to start with #85 and then #814. I believe they will be needed first. This issue is more high level and the success criteria would be passing the integration tests.