Support searching by arbitrary properties
Currently, the HTTP API https://gitlab.memri.io/memri/pod/-/blob/dev/docs/HTTP_API.md only supports searching through properties defined in the items
table: https://gitlab.memri.io/memri/pod/-/blob/dev/res/migrations/V1__initial.sql#L1
The task of this issue is, given a search API request in Pod, first search in the items
table using the indices that we have, and if they are insufficient, filter out the items manually in Rust.
Note that in the future we want to support building arbitrary indices for search
requests, but for now that is out of scope and we plan to instead do the "slow" Rust sequential filtering.