support multi-version stack/samples in registry server

[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 Server API changes required for multi-version stacks/sample support. The registry server should be able to convert new index json to the old index json struct for the existing API.
Update existing APIs to support devfile schemaVersion query.
Add new endpoints and APIs to support new GET request for multi-version stack/samples

Describe the solution you'd like

make changes to the server APIs as documented in the proposal:
https://docs.google.com/document/d/1_eXbAV1eKetKKu_FHGRw6kUUiUhcp7j1gB0uUrG0WF8/edit#heading=h.1pf53oes5yvw

Describe alternatives you've considered

Additional context