Add "additional_constraints" support

[MrBlenny]

This PR adds the ability to set "additional_constraints" in the ParameterDescription

[pac48]

@MrBlenny can you run pre-commit run -a in the root of your repository to fix the formatting?

The change looks nice overall! Is the idea of additional_constraints to pass the validation logic off to the downstream code? Since there might be domain specific validation, this seems useful.

[MrBlenny]

I'll push a commit with lint when I'm at a computer later today.

Yes, I was keen to embed JSON schema into the additional constraints and use that to generate a UI for setting advanced parameters (such as parameters that have their values as JSON encoding ROS messages). A simpler example would be to embed enum options for a strong param in the additional constraints (as JSON schema or otherwise) so you can dynamically build a UI to set params via a dropdown of enum options.

It would be pretty powerful if this library supported JSON schema as a validator but that'd of course be a big change.