Create /filter endpoint
Create /filter endpoint.

I have tried to follow roughly the same logic as the /rows endpoint.

TODO:
- [x] e2e tests
- [x] chart: only increase the number of replicas
- ~~docker-compose files~~
- [x] openapi specification
- [x] documentation pages: draft of `filter.mdx' 

Subsequent PRs:
- Index all datasets, not only those with text features. See:
  - #1854
- Complete the `filter.mdx` docs page
  - Give examples for all the supported operators
  - Enumerate all the supported column data types
- More complex validation of request parameters (to avoid SQL injection)
- Maybe rename the `search` service to `query`, with the 2 endpoints `search` and `filter`
Create /filter endpoint: Create /filter endpoint.

I have tried to follow roughly the same logic as the /rows endpoint.

TODO:
- [x] e2e tests
- [x] chart: only increase the number of replicas
- ~~docker-compose files~~
- [x] openapi specification
- [x] documentation pages: draft of `filter.mdx' 

Subsequent PRs:
- Index all datasets, not only those with text features. See:
  - #1854
- Complete the `filter.mdx` docs page
  - Give examples for all the supported operators
  - Enumerate all the supported column data types
- More complex validation of request parameters (to avoid SQL injection)
- Maybe rename the `search` service to `query`, with the 2 endpoints `search` and `filter`