Add supported method for adding new methods

[JeremyRubin]

Would be nice to have a first-class supported method for adding new methods during development.

Currently, in order to use non-positional json args you have to name a version which enables method support checking.

It would be nice to be able to either separately disable method support checking while still enabling json args or to support "patching in" rpc methods that exist in non-core implementations (e.g., those pending merge).