Reimplment the library using backends

[nrbnlulu]

Backend would replace the settings object.
we would have a default implementation for a django backend and in the future possibly other orm's.

The backed should look like this:

class GqlAuthUserProto:
	
	def get_pk_field():
		... 
	# possibly other things
	
class GqlAuthBackend(Protocol):

	def get_user_type(self) ->  GqlAuthUserProto:
		...
		
	def login(data: TBD ) -> GqlAuthUserProto:
		...
	
	# TBD

This will allow users to extend this library how ever they would want without a pile of settings.

This issue should solve

• JWT blacklist #526
• Allow modify UserType fields to return or filter extra information of the custom user #264
• Allow add extra information to token payload #265