Add a test to exercise adding canonical reference rules and pushing new
canonical tags.
A change in `rad id` was required to allow adding a new payload to the document,
i.e. one that does not already exist.
Adding rules is also documented in the man page for `rad id`.
The business logic of modifying the identity document should not live in the CLI
code for updating said document.
Instead, we extract out that logic into separate update functions and these are
using from the CLI instead.
Fintan Halpenny
Lorenz Leutgeb