lib: nrf_modem_lib: Add connection manager backend for NTN

[simensrostad]

• Add connection manager backend for NTN
  • Currently just an empty shell, content of the backend needs to be implemented.
• Add sample that showcases how to switch between cellular and NTN iface

[NordicBuilder]

CI Information

^{To view the history of this post, clich the 'edited' button above}
Build number: 1

Inputs:

Sources:

more details

Github labels

Enabled	Name	Description
	ci-disabled	Disable the ci execution
	ci-all-test	Run all of ci, no test spec filtering will be done
	ci-force-downstream	Force execution of downstream even if twister fails
	ci-run-twister	Force run twister
	ci-run-zephyr-twister	Force run zephyr twister

List of changed files detected by CI (0)

Outputs:

Toolchain

Version:
Build docker image:

Test Spec & Results: ✅ Success; ❌ Failure; 🟠 Queued; 🟡 Progress; ◻️ Skipped; ⚠️ Quarantine

• ❌ Toolchain
• ❌ Build twister
• ❌ Integration tests

Note: This message is automatically posted and updated by the CI

[simensrostad]

cc @DematteisGiacomo

Implementation shows that it is possible to switch between two ifaces bound to the same device.

Missing pieces:

• Unit test for lte_net_if_ntn
• Documentation
• Consider using separate folder for the ntn backend (right now its in lte_net_if)
• Implement the Kconfigs to support various options in the iface. NET_IF_DOWN, AUTO_START, etc...
• Copy needed functionality from the cellular implementation. PDN handling, adding IP address, etc...
• GNSS implementation
• Setting correct CFUN
• Should be straight forward to populate lte_net_if_ntn.c based on:
  https://github.com/nrfconnect/Asset-Tracker-Template/blob/gnss_ntn_usecase/app/src/modules/ntn_module/ntn.c

[github-actions]

You can find the documentation preview for this PR here.