[schema] add 'trunk' property + use it in kavlanng generator (fixes #15878)

Add an optional boolean "trunk" property for network equipement ports and channels, with no default value. In most cases, the kavlanNG generator will determine wether a link is a trunk or not by looking what the link is connected to: if it is connected to 'router' or 'switch' or 'backbone', it is a trunk, otherwise it is not a trunk. This boolean 'trunk' property allows to override this in specific cases (eg. a link to a switch in 'access' mode) It allows fixing specific corner cases (such as in bug #15878 https://intranet.grid5000.fr/bugzilla/show_bug.cgi?id=15878) with minimum refapi impact