New action: Create or update grant

This allows creating and updating grants in CiviGrant.

I programmatically grab the fields for the SpecificationBag via API4 metadata, similar to how it's done right now for custom fields. I realize that this is a naive implementation, because I don't have to specify configuration vs. parameter fields, etc. - but I think it would be great if, in time, this code was generalized to power all the "create/update entity" actions.