Suggestion: Add better callback support for addons

[Pikkupomo]

Addons should be able to provide callback functionality without declaring URL's.

This would help with the following:

• Addons don't have to provide URL's for callbacks
• Less change for duplicate URL's in the front of the store
• One place for debug

Idea for Solution

www.shuup-shop.com/callback/<identifier>

Addon would then declare callback_providers and declare my_addon in the provider:

class MyAddonCallbackProvider(CallbackProvider):
	identifier = "my_addon"

	def callback(self, request, *args, **kwargs):
		pass

after that the URL www.shop.com/callback/my_addon would call MyAddonCallbackProvider:callback