Registry index schema definition

[elsony]

The devfile registry uses a OCI based registry to host the registry content. In the registry, there is a devfile registry index file (index.json) that stores the catalog of the stacks defined in the registry. However, the registry index file currently does not have a formal schema for it.

This item is to introduce a scheme on the registry index file (index.json).

This item belongs to the parent epic: #82.

[elsony]

Renamed #135 to clarify for the registry index schema so I am setting this one as a dup of #135