Router advertisements and solicitation monitoring

Description

As a requirement coming from operating a OTBR we nee to monitor incoming RA and RS messages to react on them. It will be used to handle e.g. prefix and router changes that need to be passed on to OpenThread and other subnets.

Research if NM passes on the changes after processing incoming RA/RS in form of a dbus API or similar
Fallback to configure RA/RS handling in kernel sysctl and listen to them on upstream interfaces of the gateway
Pass this information on to OTBR, if the platform integration does not handle it already

Edited Jun 17, 2022 by Stefan Schmidt

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