[Feature] Add case insensitive alternatives for string filter options

[0xOlias]

The filter API for findMany and plural GraphQL fields (documented here) does not currently support case insensitive filtering for string column types. This is a relatively popular feature in subgraphs, often used to support search, so we should add it.

It would apply to contains, starts_with, and ends_with, and their not variants. In total, we'd have to add 6 options.

Checklist:

• Research case sensitive/insensitive comparisons in both SQLite and Postgres to determine if this is feasible, also keeping in mind index coverage
• Add new options to src/indexing-store/utils/filter.ts and tests
• Add tests in src/indexing-store/readonly.test.ts
• Update docs + write changeset