feat: Add version metric

[lcaflc]

What this PR does:

Add support for version metrics retrieved from PBS API.

Notes for Reviewer:

We have to set the version as a label because this is not a "metric". This mimic what is done with the pve_exporter https://github.com/prometheus-pve/prometheus-pve-exporter

Checklist:

• [x] I have read and understood the CONTRIBUTING and CODE_OF_CONDUCT documents
• [x] Pull Request title in the format of Conventional Commits e.g. feat|fix|chore|docs|...: Changed Something
• [x] Updated documentation in the README.md file (e.g. new parameters, environment variables, return values, ...)

[janfuhrer]

hi @lcaflc Thank you very much for your contribution! I will test your changes today.

Can you please already sign all commits (according https://github.com/natrontech/pbs-exporter/blob/main/CONTRIBUTING.md#commit-signature-verification)?

[lcaflc]

I'll sign the commits, sorry for that must check why git didn't have done it by default...