samples: common: introduce lte_monitor for nRF91

[mniestroj]

It is hard to know what is the reason of nRF91 taking sometimes so long to boot. As the LTE registration/attachment happens all in Zephyr OS boot phase (before main() is executed), there are no logs printed and nothing really indicates what is the reason of hang.

Add LTE monitor module specifically for nRF91 modem. This will log events related to LTE Link Control, such as:

<inf> lte_monitor: Network: Searching

and after connecting:

<inf> lte_monitor: RRC: Connected
<inf> lte_monitor: Network: Registered (roaming)

which will give user a hint about what happens and why doesn't the application continue to run as expected.

This also should catch all the disconnections when device has weak LTE signal and give a hint about LTE being the reason of disconnection (or reconnection) from Golioth.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 720c0ca):

https://golioth-zephyr-sdk-doxygen-dev--pr340-nrf91-lte-monito-yxx8389v.web.app

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