[Defect]: Protobuf serialization does not allow to specify bom-ref for license expressions

[andreas-hilti]

Describe the defect

The protobuf serialization does not allow to specify a bom-ref for license expressions, in contrast to the xml and JSON serialization: https://github.com/CycloneDX/specification/blob/bfb6f8baf77bbf98a4c0a54392508ba3ccf2e22e/schema/bom-1.6.proto#L368-L376 https://github.com/CycloneDX/specification/blob/bfb6f8baf77bbf98a4c0a54392508ba3ccf2e22e/schema/bom-1.6.schema.json#L1498-L1502 https://github.com/CycloneDX/specification/blob/bfb6f8baf77bbf98a4c0a54392508ba3ccf2e22e/schema/bom-1.6.xsd#L2279-L2286

Additional context

n/a