Generate bindings for functions provided by the `cshim` using bindgen.

[thomcc]

Depends on #813, so ignore the 3 patches that come from that.

Need to think a little bit about the approach here, since this is kinda hacky. It might be better done after #798, but I'm not 100% sure yet.

[thomcc]

Not sure why it passes locally but it seems that this needs to have the path to the cshim added to the include when building the cshim (this is more a note for future me than anybody else). That's not hard, I'll do that in a bit (this will need rebasing anyway after #813 lands).

[thomcc]

I'm going to say we can't actually do this one until we drop pg10 support, due to the bool differences.

[workingjubilee]

Blocked on:

• https://github.com/tcdi/pgx/pull/830

[BradyBonnette]

@workingjubilee @thomcc #830 has been merged into develop.