feat: paas-config.yaml validation

[javierdelapuente]

The 12-factor tooling accepts a configuration file paas-config.yaml that is read by the
paas-charm library. See https://canonical-12-factor-app-support.readthedocs-hosted.com/latest/reference/paas-config/

This file is written in yaml format, and currently it contains functionality about Prometheus scraping configuration and logging formatting in the frameworks.

If the file is not correct, because it is not valid yaml or because the keys/values are wrong, the application will get into error state, as it is invalid and the file has to be fixed and the charm packed again.

To improve the feedback loop, this PR will make the charmcraft pack fail if the paas-config.yaml is not correct yaml. It will also fail if the framework_logging_format is set to json for any of the frameworks where it is not supported (only Gunicorn and Uvicorn based frameworks support this option, as the other frameworks can be easily configured in the charmed application).

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• I've updated the relevant release notes.

[javierdelapuente]

@lengau can you have a look at this PR? Thanks!