Add an intrinsic to mask the "required link foo is hidden by access policy" errors

[elprans]

We currently wrap references to required links with assert_exists if the target is covered by an access policy. This is done to avoid breaking the cardinality invariant in case the access policy masks all target objects. However, in certain contexts, such as autogenerated queries (of which the UI is a heavy user) this necessitates potentially expensive workarounds (such as avoiding the link reference by computing the referenced object via a backlink combined with an explicit join).

We need an explicit escape hatch to remove the need for the workarounds. I suggest something like try() or optional(), e.g select Foo { req_made_optional := optional(.required_link_possibly_masked_by_ap) }