api: ActionUsers and ActionRoles argument value suggest

[nharraud]

It seems to me that there is currently no way to know what argument values are valid or not for a given action.

What about enabling the registration of "suggests" and "validators" in setup.py under invenio_access.actions alongside their permission?

This could be used in the CLI and the REST API.

@inveniosoftware/maintainers

Relates to #72

[jirikuncar]

We can add method/property to ParameterizedActionNeed class (e.g. iter_actions, validate_action).

[lnielsen]

@nharraud Is this a must have for v3?

[nharraud]

@lnielsen no we can work without it. We can think of a good design after the release.