feat: update Function CRD to use packageRef instead of package field

[evanwave]

Motivation

The Function CRD was using a simple package field as a string, which limited the ability to reference Package resources across namespaces and didn't follow Kubernetes best practices for resource references. This change updates the Function CRD to use a proper packageRef object that includes both name and namespace fields, allowing for better resource management and cross-namespace package references.

Modifications

• Updated Function CRD schema: Changed the package field from a simple string to a packageRef object with name and namespace properties
• Enhanced developer documentation: Added a new section in operator/DEVELOPER.md explaining the process for updating CRD definitions and ensuring Helm chart templates stay in sync
• Updated Helm chart templates: Modified the CRD template in operator/deploy/chart/templates/crd/fs.functionstream.github.io_functions.yaml to reflect the new schema structure

The changes ensure that:

• Functions can now reference Package resources across different namespaces
• The CRD follows Kubernetes conventions for resource references
• Developers have clear guidance on maintaining CRD and Helm chart consistency
• Both direct CRD installation and Helm chart installation will have consistent behavior