Allow encrypted sensitive mappings

[rbino]

If Astarte needs to send some sensitive information to devices (e.g. credentials of some sort) it would be nice to have the ability to mark some mappings in an interface as sensitive.

This could potentially do some of these things:

• Not save the value in the database (data is already encrypted during transmission, the problem is not having it encrypted at rest)
• Have some kind of initial key exchange between the SDK and Astarte which determines a shared key that is used to encrypt the sensitive data
• Use the device certificate (public key) to encrypt the data so that only the device can decrypt it with its private key

Each of the approaches above has its pros and cons, the implementation details have to be worked out if we ever decide to work on this feature