MAINTAINERS: new entry for IPv6 IOAM

Create a maintainer entry for IPv6 IOAM. Add myself as I authored most
if not all of the IPv6 IOAM code in the kernel and actively participate
in the related IETF groups.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20251014170650.27679-1-justin.iurman@uliege.be


Signed-off-by: Jakub Kicinski <kuba@kernel.org>