Implement GstDynamicTypeFactory as a new registry feature.
GstDynamicTypeFactory provides a way of registering a GType
into the registry, such that it will be registered as a dynamic
type when the registry is loaded, and then automatically loaded
if the type is needed during caps parsing.
This allows using non-core types in pad templates, by loading a
registry feature to create the GType on the fly.
https://bugzilla.gnome.org/show_bug.cgi?id=750079
