Method get_overlapping_inputs

Return all files in `level` that overlap [`begin`, `end`] Notice that both `begin` and `end` is `InternalKey` but we just compare the user key directly. Since files in level0 probably overlaps with each other, the final output total range could be larger than [begin, end] A `None` begin is considered as -infinite A `None` end is considered as +infinite

(
        &self,
        level: usize,
        begin: Option<&InternalKey>,
        end: Option<&InternalKey>,
    )