Add a mechanism for evolving JSON Schema drafts to add keywords

[Julian]

We need some generic mechanism for saying "I have a new Specification, it's like DRAFT202012 but with a new keyword foo" and then allowing the new keyword foo to either have a schema value (a la additionalItems or whatever) or schema-containing values (properties) etc...

Obviously the most "generic" mechanism will be fully defining subresources_of (and now maybe_in_subresource) as a generic callable -- but a simple/fast interface which just allows adding the extra keyword will make things easier downstream.