feat: add COS agent interface

[dariofaccin]

Description

This PR aims to add the cos-agent interface as a provider:

• Update charmcraft.yaml with new provider relation
• Update requirements
• Add integration test

Note: cos-agent interface requires Pydantic<2, so the charm config validator has been backported to work with pydantic 1.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[gruyaume]

Overall looks good. Have we tested this with the integration to grafana agent and metrics show up correctly in Prometheus?

[markbeierl]

It's going to be hard to test this until the UPF/bessd is operational

[gruyaume]

It's going to be hard to test this until the UPF/bessd is operational

Just to be clear, this is referring to this bug right?

• Processor affinity is not re-entrant safe omec-project/upf#780

[markbeierl]

Actually it was this one, where it tries to set affinity on every process, resulting in the script not running if it failed for even 1.

This is patched upstream now, as well as the patch is in our snap, so it should be good to go.

The second patch is only if you need to re-run the script, like after a bessd restart.