Add support for `commit hash` field inside the `index.json` file.

[thepetk]

/kind task

Which area/kind this issue is related to?

/area registry

Issue Description

The generated index.json file should include the commit hash of the registry repository commit is based on. This way we can compare the different versions of an index.json file and be able to update it whenever a new update is required.

Note that if the env var variable of the registry repo (to be implemented here https://github.com/devfile/api/issues/1507) is not set the commit_hash field should be empty. The new format of the index.json file should be:

{
    "commit_hash": "<hash_value>",
    "stacks": [
        ....
    ]
}

This change will have impact on the index/server side and the way that it reads the file.

Acceptance Criteria

• [ ] The commit hash that the generator was based to generate the index.json is included inside the file as a json field.
• [ ] All impacted code & test cases has been updated to facilitate these changes.