feature(barman): Add additional conf options

[ggggut]

Description

Add additional Values for configuring container volumeMounts and deployment volumes.
This will be useful if you want to have pre/post scripts in Barman (configmaps).
It templates the whole thing with toYaml.

Furthermore I added additionalConfiguration to make all parameters from https://docs.pgbarman.org/release/2.19/barman.5.html available for configuring your barman instance.
This will take a string + multiline string. So configuration would look like:

# this
  backups:
      additionalConfiguration: |
        pre_backup_script = "/path/to/your/script.py"
        minimum_redundancy = 2

# or this
  backups:
      additionalConfiguration: "minimum_redundancy = 2"

Issues

Checklist

• This PR contains a description of the changes I'm making
• I updated the version in Chart.yaml
• I updated applicable README.md files using pre-commit run
• I documented any high-level concepts I'm introducing in docs/
• CI is currently green and this is ready for review
• I am ready to test changes after they are applied and released

[hairmare]

one tiny nit: please use nindent instead of indent for consistency

other than that, this LGTM from my pov

[hairmare]

Thanks very much for the contribution!