drivers: modem: ublox SARA convert to modem context

Let's convert the SARA modem to use the more generic modem context
layers so that we don't maintain a lot of what should be shared code.

This conversion includes:
- modem context as the helper umbrella
- uart modem interface layer
- generic command handler layer
- modem socket helper
- move from net_context offload API to socket offload API

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