[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?
What behavior are you suggesting we might change?

The reason `null=True` is needed (at least for the `Coalesce` example) is because the entire purpose of `Coalesce` is to set "defaults" for null fields.

I'm not sure I believe that we'll get a significant uptick in tickets about unexpected behavior with `null=True` charfields because there are a couple used in the expressions doc examples, but I guess it's possible.

ISTM that a warning about `null=True` charfields here is a significant and unnecessary distraction from the point of the examples, which have nothing to do with forms or the admin, but are about purely ORM-level behavior.