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 makes it possible to add gateways to a multicast-group, which in
case configured will always be used for transmitting the multicast
downlinks.
This also moves the multicast class-c scheduling to the multicast-group
configuration. Options are delay between multiple gateways, or GPS time
synchronized transmission.
