Simplify external projects, unify plugin API

[jkulhanek]

This PR implements the following:

• Moves the AnnotatedDataParserConfig out of VanillaDataManagerConfig which simplifies custom methods as there won't be circular imports when DataParser is in the same file as the Model.
• Moves Annotated method configs out of nerfstudio.config.method_configs and users can extend existing configuration files in external methods.
• Unifies the discover_methods, discover_dataparsers API.