KREP-002: add declarative resource collections support

Introduces declarative resource collections to KRO, enabling dynamic
creation of multiple resources based on runtime data. This addresses the
fundamental limitation where each RGD resource block could only create a
single Kubernetes resource, requiring static definition of all resources
at design time.

This proposal adds the `forEach` field to RGD `spec.resources`, allowing
users to create N resources where N is determined at runtime from arrays,
maps, or discovered resources. The design maintains KRO's declarative
model and security guarantees while solving real world use cases like
creating resources per tenant, per namespace, or per availability zone.

While this proposal also covers related enhancements such as collection
support in `externalRef`, status field computations with collections, and
various integration points, the implementation will be split into phases.
The initial implementation will focus on core `forEach` functionality with
basic collections, followed by incremental additions of advanced features
like collection chaining and external resource discovery.

The enhancement is fully backward compatible, requiring no API version
changes or migration. Existing RGDs continue to work unchanged, and users
can adopt collections incrementally as needed. All changes are additive
and maintain KRO's existing reconciliation semantics.

Author: a-hilaly