Webhooks endpoint for updating user/groups data

[monotasker]

The webhook endpoint on the Invenio end for notifications about user data updates will be at /api/webhooks/idp_data_update. It supports GET to simply confirm the endpoint is listening (200 response) and POST to send a signal (202 response if successful).

As we discussed, this just sends notices that updates have happened. Invenio will then query a commons endpoint for the new data.

I've got it set up so far to receive JSON signals with the format below. This allows sending multiple signals in one POST request. It also allows sending signals about updates to group metadata as well as users' individual data.

[monotasker]

Possible signal content format:

{
    "idp": "knowledgeCommons",
    "updates": {
        "users": [{"id": "1234", "event": "updated"},
                  {"id": "5678", "event": "created"}],
        "groups": [{"id": "1234", "event": "deleted"}]
    }
}