dynamic types: Implement dynamic types in the registry

dynamic types: Implement dynamic types in the registry

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