[Variant] Avoid second copy of field name in MetadataBuilder

[alamb]

• part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736

The observation is that MetadataBuilder has both a vec and a BTreeMap. Each field name is stored twice, once in the Vec and once in the BTreeMap.

arrow-rs/parquet-variant/src/builder.rs

Lines 235 to 237 in 674dc17

	struct MetadataBuilder {
	field_name_to_id: BTreeMap<String, u32>,
	field_names: Vec<String>,

I think the idea is if we used an IndexSet we would not need the second copy of the field

If I'm understanding correctly...

• We need a map (not necessarily ordered) in order to cheaply find the field id for a given field name.
• We need a vec that remembers insertion order?
• The map cannot reference strings in the vec, unless we're willing to mess with interior mutability and "fun" like that (but in theory we could use a hashmap that stores indexes into the vec, with a fancy custom indirect hasher)
• The vec cannot reference strings in the map, because there's no stable way to reference the strings it hosts.

Have we considered using an IndexSet? An IndexSet<String> should behave like a Vec<String> but with O(1) cost to return the index of any entry.

Originally posted by @scovich in #7795 (comment)

[alamb]

label_issue.py automatically added labels {'parquet'} from #7795