adds new interface for members' membership providers to implement so we know what aliases to store data against.
Merge remote-tracking branch 'origin/6.2.0' into 7.1.0

Conflicts:
	src/umbraco.providers/members/UmbracoMembershipProvider.cs
Updates the underlying UmbracoServiceMembershipProvider to deal with 'object' for provider key, this then lets us deal with either guid or int for the new membership provider, just like the old one does.
fixing some casting
Merge remote-tracking branch 'origin/6.2.0' into 7.1.0