Add index generator support for registry github URL source.

[thepetk]

/kind task

Which area/kind this issue is related to?

/area registry

Issue Description

Atm the index/generator is only supporting a filepath of a local registry dir in order to get all the info and generate the index.json file.

This implementation will help us In order to avoid copying all the files during the build of the devfile-index image and to be able to support dynamic stack updates (as mentioned in the parent EPIC).

Acceptance Criteria

• [ ] The index/generator supports using github URLs as the source of the registry and generates the index.json from data fetched directly from github.

[github-actions[bot]]

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