Define python level abstraction for a Bundle

Is your feature request related to a problem? Please describe.
Currently, we developed many bundles in the model-zoo repo and developed the ConfigParser to load configs.
But actually we don't have a clear definition about what a Bundle is in the python level.
It would be nice to define the Bundle class to represent a bundle and enable users to follow & define their customized bundle structure, and provide the APIs for other applications to easily load and handle a bundle, like MONAI Label, MONAI FL, etc.
Several draft steps to do:

• Develop BundleWorkflow interface and implement config-based subclass, so bundle can be implemented with JSON/YAML config or python code, just need to follow the interface.
• How to expose the CLI entry and args for customized BYOCBundleWorkflow in bundle/scripts/.
• Update FL monai algo to use new workflow.
• #6720
• Add tutorial to show how to develop a new pure python-based bundle.
• Develop Bundle interface for the whole bundle, mainly about folder structure, files, download, etc.