By registering functions directly we cut off the dependency of the
"resource extend" functions on the plugin. This is a step towards
the goal of removing the CommonDbMixin mixin class.
Also, we register all "resource extend" functions at plugin create
(in __new__) instead of in the class definition (which caused the
hooks to be registered on import). This ensures the "resource
extend" functions are only registered for the plugins/mixins that
are actually used.
Note that decorators are used to register "resource extend" methods,
similar to the callback receiver decorators.
Related-Blueprint: neutron-lib
Change-Id: I128cfda773d5f9597df9cd61261fdc05f2a174aa
b3c0d5f239