Three-tier preload configuration for a relation Field.
Supports separate preload lists for all users (always), master
users (master), and tenant users (tenant), allowing fine-grained
control over which relationships are eagerly loaded based on the
current user's access level. Each list accepts atoms or
{source, alias} tuples — the :always list is always read; the
:master and :tenant lists are merged in based on
state.master_user?.
Mirrors the resource-level MishkaGervaz.Form.Dsl.Source
preload block but scoped to a single relation field.