drivers: modem: sara-r4: add offloaded DNS handling

SARA-R4 modem supports offloaded DNS via AT+UDNSRN command.
Let's implement it.

NOTE! On SARA-R4 a new firmware *IS* required to support this feature:
L0.0.00.00.05.08 [Apr 17 2019 19:34:02]

Signed-off-by: Michael Scott <mike@foundries.io>