Allow PKI message MQTT downlink for 0 hop neighbor nodes.

[esev]

Currently it appears that downlinked PKI messages are only transmitted when both the From and To nodes are in the nodedb.

firmware/src/mqtt/MQTT.cpp

Line 200 in ae4f542

// Only accept PKI messages to us, or if we have both the sender and receiver in our nodeDB, as then it's

Would it be possible to also transmit the message when the To node is a direct 0 hop neighbor?

If I'm outside the range of my local/normal mesh it'd be nice if a PKI direct message could still be transmitted to me by a nearby MQTT gateway node (assuming the sender enabled the OkToMqtt bit).

Furthering the idea: Could the Neighbor Info module both transmit using a hop limit of 0, and keep a record of the Neighbor Info records from its 0 hop peers? If so, then the MQTT module could rely on the Neighbor Info module to provide both 0 hop and 1 hop peers and use that information to determine which PKI messages to queue for transmission.