Export OSV database

[Tristan de Cacqueray]

On advisory merge, we could generate the OSV version and push it to a branch, similar to what rustsec is doing with: https://github.com/rustsec/advisory-db/blob/main/.github/workflows/export-osv.yml

• Implement a converter to transform the Advisory data type into the desired JSON object
• Add a subcommand to the hsec-tools to convert one md file
• Implements a GitHub workflow action to publish to a osv branch

[Fraser Tweedale]

This has been implemented in #78.