Turn scan storages into plugins #6603
Description
Scan storages (storage readers / writers) also seem to be a good fit for being implemented as plugins. They should straight use the new org.ossreviewtoolkit.plugins.api.Plugin API.
Some additional notes:
- Ideally, this would also address Allow scan storage providers to express that they only work for packages (not projects) #6460.
And Scanner: Storing the scan result via HTTP storage backend fails with WebDAV on a Caddy-based HTTP file server #6765 would probably be a good test case for a new plugin.