support multi-version stack/samples in registry generator

[yangcao77]

Which area this feature is related to?

/area registry

Which functionality do you think we should add?

Why is this needed? Is your feature request related to a problem?
sub issue for #735

Detailed description:
Registry Generator API changes required for multi-version stacks/sample support. The registry generator should support both old and new stack repo struct, should be able to parse and validate the stack.yaml file, and generate index.json with the new schema struct

Describe the solution you'd like

make changes to the generator APIs as documented in the proposal:
https://docs.google.com/document/d/1_eXbAV1eKetKKu_FHGRw6kUUiUhcp7j1gB0uUrG0WF8/edit#heading=h.ggwv8nowatl2

Describe alternatives you've considered

Additional context