This adds back support for Synology DSM.
The implementation is compatible with DSM 7.2+ and integrates with DSM to forward requests through the DSM server into Duplicati.
The setup features database locks, using a random auth-key so DSM handles authentication.
The database is encrypted with a random key, and all data is stored in the `var` folder.
The uninstall can optionally remove the `var` data.
This is still a work in progress. The authentication integration with DSM does not currently work.
Fixed some issues with the manifest urls.
Changed to records and nullable in some places.
Regenerated the test-keys to use RSA208.
Changed to using built-in JSON for manifest.
This fixes#5145
Kenneth Skovhede