Feature 10887: Add cross-model relations support

Changes:
- Extend `add_relation` method in N2VCJujuConn to include the CMR case
- Add `add_relation` method to K8sJujuConn
- Add n2vc/definitions.py file that includes definition ofjects for
Offer and RelationEndpoint.
- Change `n2vc.libjuju.Libjuju.list_offers` method to be private, and accept a filter `offer_name` parameter.
- Update `n2vc.libjuju.Libjuju.consume` method arguments.
- Add `n2vc.libjuju.Libjuju.offer` method to create an offer.

Unit tests associated to the code changes have been either created or
updated accordingly

Change-Id: Ibf8d574528dee0fa898e0e97578dd3a6aa68316a
Signed-off-by: David Garcia <david.garcia@canonical.com>