A0-544: prepare alerts backup

[woocash2]

Introduce BackupItem - an enum which encapsulates different types of data written to backup. Briefly, backup is now prepared to contain:

• units
• own alerts
• network alerts
• multisigs

The split between own and network alerts is made because we want to perform different steps for them after having them saved in the backup.

Adding BackupItem required to adjust runway and test cases for backup.

In this pr, alerter doesn't interact with the backup yet.

[github-actions[bot]]

Please make sure the following happened

• [ ] Appropriate tests created
• [ ] Infrastructure updated accordingly
• [ ] Updated existing documentation
• [ ] New documentation created
• [ ] Version bumped if breaking changes