Keep type ids in Union datatype to follow Arrow spec and integrate with other implementations

[viirya]

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

Related to #1404, generate_unions_case integration test case.

We keep a vector of Field and a UnionMode in Union datatype now. According to Arrow spec, it only specifies the type ids are 8bit signed integers. So the type ids can be arbitrary integers only if they can be matched with children arrays' type ids. This is how UnionType in C++ implementation works.

Currently this crate assumes that the type ids of Union are monotonically increasing from 0 (i.e., 0, 1, 2...). When integrating with other implementations like C++, this assumption will cause error in receiving/sending UnionArray.

Describe the solution you'd like

We should keep type ids in Union datatype like C++ UnionType. The type ids are given when creating Union datatype. UnionArray related code should not assume the type ids are monotonically increasing from 0.

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[viirya]

I will work on this and propose the PR later.