There are 2 concurrent models, this one is an historical legacy. The type to use the one
from `libervia.backend.models.core`, and it will progressively be used everywhere and
fully based on Pydantic.
Implementation of autocrypt: `autocrypt` header is checked, and if present and no public
key is known for the peer, the key is imported.
`autocrypt` header is also added to outgoing message (only if an email gateway is
detected).
For the moment, the JID is use as identifier, but the real email used by gateway should be
used in the future.
rel 456