Zephyr: Driver: modem helper should make it easier to implement a modem

[mike-scott]

Is your enhancement proposal related to a problem? Please describe.
The current modem receiver driver is pretty bare bones. It does the UART buffering in a ring buffer and allows the modem drivers to pull data in nice chunks. It does not abstract the command parser, sockets to communicate between the modem and Zephyr, pin config settings, etc. What this means is that each modem has quite a lot of copy/paste code atm.

Describe the solution you'd like
I'm proposing to implement a new modem helper driver called "modem context". This would include the majority of the command parser, some pin config and a better abstraction between the offloaded sockets layer and the modem. The release after the modem context is merged, we can depreciate the modem receiver driver.

Describe alternatives you've considered
I considered updating the modem receiver driver, but felt there might be some users who want more time to move to the new modem context helper.

[mike-scott]

I plan to submit a PR for this before August 9th feature freeze cut off.

[mike-scott]

Adding a few people who have been interested in modem work:
@weinholtendian
@WilliamGFish
@rerickson1
@cybertale

[mike-scott]

@Oanerer

[mike-scott]

@Swij