[Feature] Webhook URL for user creation

[fmigneault]

Add a step just before user-creation completes, but after verification (unique, valid name, etc.) that sends a request to desired location for 'extra-steps'. Should be accomplished in create_user, just before completion, around here :

Magpie/magpie/api/management/user/user_utils.py

Lines 97 to 99 in 2177890

	return ax.valid_http(http_success=HTTPCreated, detail=s.Users_POST_CreatedResponseSchema.description,
	content={u"user": uf.format_user(new_user, new_user_groups)})

To avoid race conditions from the webhook response vs other same user-name request, we could commit the changes before waiting for the webhook reply. That username would then be reserved. To keep it as 'pending', set its status=0.

Request:

POST {MAGPIE_WEBHOOK_PRE_USER_CREATION_URL} 
{"user_name": "<the-name>"}

When not provided by config, this step is simply skipped.
Handle response from this request

• When 2xx (handle OK/Created/NoContent): complete user creation as normal
• Else: cleanup pending user + return 424 Failed Dependency

[fmigneault]

resolved by #378