Support upgrading to candidate or edge channels for testing

[samuelallan72]

In cases where we want to test an upgrade of a prerelease, we would need support to upgrade to candidate or edge channels of charms. Currently this is hardcoded to stable channels in COU.

We could introduce a flag to COU plan and upgrade to allow upgrading to a different track:

$ cou upgrade --upgrade-to-track candidate

Design thoughts on such a flag:

• it must default to stable
• it should be an enum type, limited to stable, candidate, and edge
• it should be documented on it that it's not recommended to candidate or edge on production clouds.

[jneo8]

Instead of provide a way to override the channel global. Maybe it will be more useful we allow user to decide which target channel for a single app upgrade to.

For example, we tend to keep mysql-innodb-cluster in channel 8.0/stable but want to test keystone on 2024.1/candidate.

The solution will be we follow the way how we create https://github.com/canonical/charmed-openstack-upgrader/blob/main/cou/utils/openstack_lookup.csv to create another csv file for the target channel and allow user to choose their own modify version.