Add display helpers

I have a couple of ideas for a `display` submodule:

### Backup tables

Since we should have more or less the same fields in our charms, we could regroup our existing solutions to create those kinds of tables:

```
backup-id | backup-type | backup-status
----------------------------------------------------
YYYY-MM-DDTHH:MM:SSZ | logical | finished
```

Two existing solutions:
- Manual approach: https://github.com/canonical/mongodb-operator/blob/6/edge/lib/charms/mongodb/v1/mongodb_backups.py#L576
- Object oriented (with third-party dep): https://github.com/canonical/zookeeper-operator/blob/main/src/managers/backup.py#L160

### Log friendly state diffs

We work with data structure subject to changes. It can be a relation databag or an application configuration.
The idea here is to create a function to properly display the difference between a "before" and an "after" state.
For reference, here is what I like to see in my package manager post upgrade:

```
Version changes:
[U.] #1 pyenv 2.4.11 -> 2.4.12
[U.] #2 pyright 1.1.378 -> 1.1.380
[U.] #3 pyright-internal 1.1.378 -> 1.1.380
[U.] #4 ruff 0.6.4 -> 0.6.5
[U.] #5 uv 0.4.8 -> 0.4.10
[U.] #6 zoxide 0.9.4 -> 0.9.5
Added packages:
[A.] #1 bottom 0.10.2
Removed packages:
[R.] #1 btop 1.3.2
```

I think ^this (replace "packages" by "keys") would be a nice addition to log state changes in our charms.
Pedro made a valid point by saying that we need to scrub passwords from those. We could take some inspiration/regex from what sos report does?

Start by comparing the referenced backup implementations in mongodb_backups.py and backup.py, then determine the package entry point for the proposed display submodule. Define the backup-table and state-diff interfaces, including password scrubbing, and add tests that cover the displayed output and sensitive values.

Written by the indexing model from the issue text.