[Backport 14.7] In the PMA v1.0 response, do not stop serializing referenced docs if the doc is an asset or imageset

Note : for the backport, we only need to backport the PMA part from ENT-3369 since the CMA is not supported in PaaS

Customers run into

_maxreflevel

issues where the pagemodelapi does not follow references deeper than 1 level. Increasing this level is very tricky because the PMA will become even more verbose and might even blow up completely.

However, it is better if we do not 'truncate' the max level when the reference is to an imageset or asset, such that even if the imageset or asset is at level 2, we still include it in the PMA : the json of an imageset / asset is always small any way and in general when a doc has a reference to, say, an imageset, it is really needed in general.

Note this is not a silver bullet improvements as there is no for the PMA: in the end, it is a balance between being verbose and chatty.

This feature is not replacing the need for the content delivery api at all of course

Note: we only support this improvement for v1.0