This makes it possible to automatically remove items from the queue in
case the expires_at timestamp has reached. This field is optional and
the default remains to never expire queue-items.
This implements end-to-end encryption between the end-device and
end-application. The encrypted AppSKey or SessionKeyID is forwarded to
the end-application which should be able to decrypt or request the
AppSKey to decrypt the uplink payload. As well the end-application will
be able to enqueue encrypted application payloads.
Using this mechanism, ChirpStack will never have access to the uplink
and downlink application-payloads.
This reduces the amount of dependencies in case not all features are
being used. E.g. tonic is only needed if using gRPC and pbjson,
pbjson-types and serde are only needed if using the JSON serialization.
