Add signing capabilities

[amyramos]

Is your feature request related to a problem? Please describe.
Maybe it will be a great feature to have an opportunity to add signatures to data stored in the database. So, it will be possible to verify integrity of data stored in the database on the client side. It can be used to determine if there were changes of the signed data.

Describe the solution you'd like
Add a new structure called SignedValue which will use SignatureAlgorithms variable and as EncryptedValue use the first algorithm to sign data and use other algorithms with lower priority (fallbacks) if it's not possible to verify signature with the first (primary) algorithm. Example SignedValue struct based on EncryptedValue:

type SignedValue struct {
    Raw   interface{} `json:"Raw"`
    Valid bool        `json:"Valid"`
}

Raw is value which is stored in the database, Valid is boolean which tells if the Raw value is signed correctly.

[amyramos]

@pkosilo, I'd like to work on it myself

[pkasila]

@pkosilo, I'd like to work on it myself

Okay 👍