net: lib: coap: Add new API to configure retransmission settings

Add new functions to the public CoAP API to configure CoAP packet
retransmission settings. Application may need to re-configure the
settings for example when cellular modem changes connection from
LTE-M to NB-IoT or vice versa.

Signed-off-by: Juha Ylinen <juha.ylinen@nordicsemi.no>