Adding "change summary" to the diff response for common prefixes in the API

[iddoavn]

It will be helpful to add add the "change summary" to the diff response for common prefixes in the API, something like this for example:

{
    "path": "metadata/illegal-queasy-twist/",
    "path_type": "common_prefix",
    "type": "changed",
    "change_summary": {
        "changed": 3,
        "added": 999,
        "removed": 1
    }
}

Giving a similar summary of what you can get out of thr UI.

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