Authenticate storage endpoints (?)

[adewes]

Currently the storage endpoints are unauthenticated. This is good for privacy as the backend cannot identify the encrypted data of users. On the other hand it's also a risk as data can be retrieved by anyone with knowledge of the ID and as malicious users could overwhelm the storage backend.

We could authenticate users via their key (providers) or their token (users) and only allow them to store and optionally only allow them to retrieve and store data from/to specific IDs. This would alleviate the risk of malicious users overwhelming the storage backend, but it would also make encrypted settings identifiable by the backend.