Add some basic linting

[di]

While most advisories are automatically generated, occasionally they will be hand-written (e.g. https://github.com/pypa/advisory-database/pull/72/).

It would be nice to add some basic linting that ensures files are correctly formatted and have the minimum required fields, and run this on a new PR.

[woodruffw]

OSV provides a JSON schema here, so validating against that wouldn't be too hard 🙂

[oliverchang]

+1 to this! A JSON schema based validator should be trivial.