[Feature]: backup list readable through CNPG cli

### Is there an existing issue already for this feature request/idea?

- [x] I have searched for an existing issue, and could not find anything. I believe this is a new feature request to be evaluated.

### What problem is this feature going to solve? Why should it be added?

Exact status of the backup store can be complex for tools like pgbackrest and contain more info than is possible to store in Backup CRs.

### Describe the solution you'd like

Add another backup (and WAL?) capability that would specify support for a backup list/status endpoint. Such endpoint can then be used by CNPG during `kubectl cnpg status` calls in either the **Continuous Backup** section (which seems to be empty when using a plugin) or even under a dedicated command (maybe `kubectl cnpg plugin-status `?) which would allow plugin to provide a custom, plugin-specific output.

### Describe alternatives you've considered

Alternative is to use the tool directly to check status but that's not easy unless config is stored locally (or at least in the container).

### Additional context

During development of the [pgBackRest plugin](https://github.com/operasoftware/cnpg-plugin-pgbackrest) I’ve noticed there is no easy way to show users output of the `pgbackrest info` command. Note that in pgbackrest there may be multiple storage targets connected to a single instance and each storage target may contain stanzas (backups and WAL streams) from multiple databases.

### Are you willing to actively contribute to this feature?

No

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct