Support row query api

Bring it over from entity if possible.

- [ ] Implement API based on previous work with [`entity-rs`](https://github.com/chipsenkbeil/entity-rs)
- [ ] Add indexing support for faster lookup based on equality of string, number, etc.
- [ ] Create derive macro to produce a typed query

Start by reviewing the existing memtable-rs code and the linked entity-rs project to identify the prior row-query work that could be carried over. Completion requires a row query API, equality-based indexing for supported values, and a derive macro for typed queries, as described in the checklist.

Written by the indexing model from the issue text.