UnionBuilder Create Children With Capacity

[tustvold]

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

UnionBuilder is created with a capacity, but this is only used to pre-allocate its type_ids buffer. The child buffers are created with a capacity of 1.

Describe the solution you'd like

We should save this capacity and pass it to FieldData::new. In the case of a sparse union, we should pass the maximum of the currently length and the capacity.

Describe alternatives you've considered

Additional context

[kastolars]

Hi, has anyone started on this issue?

[tustvold]

No, would you like me to assign it to you?

[kastolars]

I can give it a shot yeah, I'll try to get a PR out today.