Custom Auxiliary Functions for SQLite

[Boscop]

Similar to SQLite custom functions (#1103) which diesel now supports (#1691), SQLite also has the related concept of Custom Auxiliary Functions for its full-text search (FTS) functionality.

Implementing a custom auxiliary function is similar to implementing a scalar SQL function.

It would be useful to be able to register a Rust function with sqlite in the same way as normal sqlite custom functions.

[weiznich]

We are happy to receive a concrete proposal for such a api + some thoughts about how to handle this for sqlite distributions with FTS disabled. Otherwise this is at least for me nothing I plan to work on for a foreseeable future.