feat: add custom metadata support

[matmair]

Description

Implementation of the feature discussed in #660
This adds 2 settings and control flows that allow the addition and validation of custom data to the metadata file. This way it is easy to ensure the environment your restore in matches the condition needed for a restore.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[matmair]

@Archmonger this would be ready for a first review; is this in general to this projects standards, and what should I change / adapt?

[matmair]

@Archmonger anything you want to see added/removed here?

[Archmonger]

Code needs to be formatted via hatch fmt dbbackup/

Can be merged after addressing these remaining comments.

[matmair]

I think all comments should be addressed now; sorry about missing the formatting in contribution, I did not find a way to get hatch run lint:format working

[Archmonger]

Apologies for that. This repo does not have a hatch run lint:format command. That seems to be a copy-paste error from another one of my repo's contributing.md.

All Python linting for dbbackup is performed via hatch fmt <directory>.

I'll review/fix the contributing docs in a follow up PR.