Find (or select) split metastore API?

[guilload]

I've noticed this pattern a few times in the codebase:

metastore.list_all_splits().iter().find()

In the short-term, we could expose a new find_split(index_id, split_id) -> Option<Split> metatore API. In the long-term, we probably want select_splits(index_id, expression) -> Vec<Split> where expression can express arbitrary filters.