Record a dependency when a field refers to another resource

Providers resolve cross-resource references themselves, through fields
like vpcIdSelector, so Crossplane never sees the relationship and can't
order anything. Writing the reference as a field value instead makes it
visible: the function says where the value comes from, and the SDK
records the dependency.

reference.named names a composed resource, and named_required a
required one. Reading a field through it records a path, and ref turns
that into a marker string; external_name references the external name,
which is what a selector would have resolved to. reference.resolve then
replaces each marker with the observed value and records the edge.

A marker is a string because that's what a generated model's own
validation accepts in a string field, so it survives model_validate and
resource.update untouched. Given the model, the stand-in follows the
model's fields: a typo fails where it's written, and a field whose JSON
name is a Python keyword (from, class) is recorded under its JSON name.
It has no value, so a truth test or str() raises rather than taking the
wrong branch the way an always-truthy placeholder would.

A reference whose source doesn't exist yet leaves the field out, and
the edge means Crossplane doesn't create the resource until it does. A
resource that already exists keeps the value it has, so an apply
doesn't unset it. On a Crossplane without CAPABILITY_DEPENDENCIES the
edge would be ignored and the resource created without the field, so
there a resource with an unresolved reference is left out instead.

The runtime fails a response that still contains a marker, so
forgetting to call resolve can't send one to the cluster.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: stevendborrelli