Implement a reduce API

[ecton]

The last major feature for BonsaiDb that we need is to consume the new features implemented in #19 and #21 is the ability to compute a reduced value over a partial scan of a tree. Originally when implementing the underlying features, I envisioned "reduce" being a BonsaiDb views feature, not something that Nebari itself would offer. However, upon more thought, it definitely would be a great feature to have in this crate.

The API should look something like this:

impl TreeFile {
    fn reduce<R: RangeBounds<Buffer>>(&self, range: Buffer) -> Root::ReducedIndex {}
}

That signature is pseudo-code, and probably requires additional parameters.