net: ipv6: Allow user to configure the IPv6 MTU size

This makes it possible to set the minimum IPv6 packet size that
can be sent without fragmentation. The default value is 1280 bytes.
This commit allows user to set the IPv6 MTU value within reasonable
limits [1280, 1500].

Fixes #61587

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>