[DPE-4408] Add differential Backup support

[lucasgameiroborges]

Issue

Link to issue: https://warthogs.atlassian.net/browse/DPE-4408
Add support for differential backup, plus a new parameter type to create-backup action for selecting the backup type (full or diff).

Solution

• Implement new param in create-backup juju action.
• Include the backup type in list-backups action output.
• Refactor to make backup charm logic handle different backup types (label formatting).
• Adapt integration test to try both full and differential backups.
• Small fine-tuning in integration and unit tests.

Follow-ups

• Update documentation to mention differential backups: https://warthogs.atlassian.net/browse/DPE-4357
• Investigate the current main source of instability in integration test: https://warthogs.atlassian.net/browse/DPE-4358
• Replicate this PR to PG VM: https://warthogs.atlassian.net/browse/DPE-4389