Refactor TopKHashTable to use HashTable API (#19464)

## Which issue does this PR close?

Part of #13433

- [x] Add some docs
- [x] Report performance changes

## Rationale for this change

Getting rid of `RawTable` so we can freely upgrade `hasbrown`, also
removing a lot of unsafe code at the same time (helping with later
refactoring / optimization as well).

Note to reviewers: this change was in large part done by Google Jules
(Gemini), I mainly did some review / cleanup.

The (micro) topk aggregation benchmarks show no difference on my machine
(as desired)

## What changes are included in this PR?

Changes storage to use a separate `Vec` to store index-based info
instead of directly in hashmap.
## Are these changes tested?

## Are there any user-facing changes?

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Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: Dandandan