Ensure Secret.id includes model UUID

[benhoyt]

Currently Secret._canonicalize_id adds the secret: prefix if it's not there, but it doesn't add the model UUID. It would be helpful if it added the model UUID too, especially for cross-model relations. See more information at https://bugs.launchpad.net/juju/+bug/2075153

# Current logic:
id if id.startswith('secret:') else f'secret:{id}'

# Change it to something like:
id if id.startswith('secret:') else f'secret://{model_uuid}/{id}'