I envision the following command:

ipa <obj> add-schema --schema <file>

Obj can be "user", "group" etc. We can start with just user.

The file will have an object class and attribute definition.
The utility will:
- Check that schema is correct
- Make sure it does not have mandatory attributes
- Load this object class into the schema
- Add object class to the list of object class definitions for the object (currently we have one just for the user entry)

The UI will read this definition and offer the new attribute in the UI during modify operation.

In future we might also add something like:

ipa <obj> schema-suspend <objectclass>

that would remove the object class from the list of definitions so that it is not offered any more for addition/modification in UI.

Ticket has been cloned to Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1224061