Publicly export MapFieldNames

[alanhe]

Describe your question
arrow::array::MapBuilder is public (#1355), but MapFieldNames is not, making it impossible to call new() and with_capacity() with a proper value.

arrow-rs/arrow/src/array/builder/map_builder.rs

Lines 61 to 68 in 3096591

	pub fn new(
	field_names: Option<MapFieldNames>,
	key_builder: K,
	value_builder: V,
	) -> Self {
	let capacity = key_builder.len();
	Self::with_capacity(field_names, key_builder, value_builder, capacity)
	}

I guess MapFieldNames should be exported too, should it?

[tustvold]

This was an accidental bug introduced by #1879

[alamb]

@tustvold is this a regression in 19.0.0 (aka should I cut a new release candidate)?

[tustvold]

I think it was a regression that landed in 17.0.0 and was only just noticed...