json schemas for yaml files

[sed-i]

metadata.yaml, actions.yaml and config.yaml would be much easier to write with auto completion, and charm authors would encounter less runtime errors with auto validation.

I created a json schema for metadata.yaml based on the Charm Metadata v2 post by @hpidcock and I really like the autocompletion + validation IDEs offer once you have a schema (I tested the schema using vscode + yaml extension).

• Validation could be done as part of charmcraft build (@facundobatista)
• Auto completion could be achieved by pointing the IDE to the json schema locally, and eventually, perhaps, via the schema store.

Schema draft: metadata-schema.json

[facundobatista]

If there are "official" schemas, we can add checks at build time in charmcraft, for sure.