Simplify SubjectMapping cluster discovery with unified clusters field

Replace placementRef with a unified clusters field supporting two types:
- InferFromServiceAccount: Auto-discovers via ManagedClusterSetBindings
  in ServiceAccount's namespace (ServiceAccount subjects, default)
- ClusterList: Explicit cluster names (User subjects, required)

This simplification removes the Placement API dependency and provides
a cleaner, more consistent design for Alpha. Additional cluster
discovery types (ManagedClusterSet, ClusterSelector, Placement) can
be added in future releases.

Key changes:
- Add clusters field with type discriminator to SubjectMappingSpec
- Remove placementRef field and PlacementRef type
- Update validation rules for subject-specific cluster requirements
- Simplify API examples and cluster discovery documentation
- Move advanced discovery types to Future Enhancements section

Signed-off-by: Jia Zhu <jiazhu@redhat.com>
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: zhujian <jiazhu@redhat.com>

Author: zhujian7