Data model queries

[CasperWA]

Some thoughts from discussions with @jesper-friis.

The query functionality for driver plugins might be useful when moving to larger "dump" databases for data models to be used by a wider userbase. To implement this, it could be nice to have an established query language as the DLite-facing query, e.g., GraphQL, and then implement the proper backend/driver-specific query within the appropriate method.

I suggest here GraphQL, because it matches well with the data structure and format used by DLite, as well as being widely supported, which should result in easy implementation for the various existing and new drivers/backends.