New flavor of injected expression

[zachmu]

No description provided.

[fulghum]

InjectedExpr gets a bit more complicated to use though. Seems like you could probably get away with adding OrderBy directly to InjectedExpr, too, and avoid the new type, but I understand having the type makes it convenient to switch on type. The biggest design point I can think of there is if we think we'll end up adding many more of these extensions, and if the extensions will have cases where we need an OrderBy and some other field that comes from a different extension type. That ends up getting messy with many similar types obviously.

Overall seems reasonable though, and it sounds like we want to revisit aggregation in the future as we better understand what Doltgres needs, so I'm on board.