Add fromJson method in DeltaCRDT
DeltaCRDT should provide a fromJson method allowing the deserialization of crdts without knowing its exact underlying type. This would be particularly useful in the c-service witch manipulate DeltaCRDT.
Should use a switch case to call the correct underlying fromJson method. Even if this seems not very clean, it avoids to force users to do the exact same code outside the library.