fix: not imported Source / Element classes are not registered

[mhordynski]

Source subclasses are automatically registered upon import when declared. This enables us to automatically map them from JSON to the correct subclass or use a URI path to determine which implementation should be used (for example, gcs://something routes to GCSSource).

Currently, if a given implementation is not imported, this feature does not work. The goal of this task is to provide an explicit method to register source classes.

[mhordynski]

On core level of our configuration in pyproject.toml we should add new section, where we specify module paths what should be imported via ragbits.

[mhordynski]

Same mechanism will work for Element objects - need to document that.