Define strings related to encoding a device message
in JSON.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

Add a general purpose JSON message encoder,
nrf_cloud_encode_message().

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

When encoding the shadow, let the caller determine if
the reported object should be included.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

Split out update function so it can be used for CoAP.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

This was incorrectly using the ++ operator when atomic_inc()
would be better.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

Modem sec tags are unsigned, so use that representation
instead of plain int.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

Uses the Zephyr coap_client library. Sends and receives
data with nRF Cloud using CBOR and sometimes JSON.

Supports most nRF Cloud services - FOTA, data messaging,
location services (cellular, Wi-Fi, A-GPS, P-GPS).

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>

Document the new nrf_cloud_coap library.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>
Co-authored-by: Pekka Niskanen <pekka.niskanen@nordicsemi.no>