Add ICMPv6 Destination Unreachable sending for Border Router (SPEC-1218)

[EskoDijk]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is.
Problem stated in https://threadgroup.atlassian.net/browse/SPEC-1218 :
a Thread Device that only has a ULA OMR address (BR-generated prefix), may try to contact a cloud server continuously even though it will never work with such address type. This happens in case the BR publishes a ::/0 prefix on the mesh that suggests the cloud server is reachable, while that ::/0 only is meant to communicate reachability of a non-ULA-prefix on the AIL.

Similarly, regardless of Thread Device address type, if the BR doesn't have a route to the destination, but as before the ::/0 route was published, the message will be dropped but the sender not notified - sender may assume congestion, so may keep trying forever.

Describe the solution you'd like A clear and concise description of what you want to happen.
As proposed in SPEC-1218: BR to send ICMPv6 Destination Unreachable message in such cases.

• If there's no route possible to destination, Code 0 can be sent
• If there's a route possible, but BR knows destination is not on the AIL, so not reachable using the ULA OMR address, Code 1 can be sent (see SPEC-1218)

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.
See SPEC-1218

Additional context Add any other context or screenshots about the feature request here.
I didn't check the code in detail on what is already implemented with ICMPv6 error messages - but it seems there's only some code related to ICMPv6 errors for on-mesh destination and not for this case.