[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?
The problem is MySQL. You can use "--column-type-info" for mysql command line client to see that the return type of the expression is blob. Of course, this being MySQL adding a CAST as char in there doesn't actually do anything. The return type is still blob. You can use "cast(concat(a, b) as char(10000) character set utf8)" to get a real cast as VAR_STRING, but you'll have to supply the length, too. And, this being MySQL you'll get silent truncate of data if the length is too short...