[WIP] Fixed #23753 -- Added a suite of SQL Functions

This is a work in progress, but not too far off being code complete. I thought it was better to open a PR rather than have people try to track individual commits on my fork.

A few notes at this point in time:

- ~~I plan on renaming Concat to ConcatPair (name TBD), and renaming ConcatMany to Concat.~~
- Some backends allow an arbitrary number of args to Concat, but I've chosen implementation simplicity by implementing multi-concat the same across all backends. If performance differences can be shown, I'll revisit this choice.
- ~~Some functions validate args in init, others in resolve expression. I think I'll standardise on init, since objects should be fairly complete on instantiation.~~
- I won't be implementing Case/When just yet. Michal on the ML has a better implementation than the one I had prototyped, and I'll let Michal finish and submit a PR for that part.
- ~~Tests are basic and should probably cover a lot more failure states.~~
- ~~Docs. They'll come last as always.~~

Are we happy about the namespacing? Is `models.functions.*` an OK place?
That was the first thing I tried - but converters only apply to the outer-most expression. Inner expressions do not have their db_converters run. I could make db_converters return all inner converters too, so that inner runs before outer. I think that's actually the correct thing to do.

But I'd still like to track down why bytes are returned for this expression but not for TextFields. I'll do that separately though.

I think the signature change makes sense as it offers a lot more power to third party backends.