Feature request: Support multiple annotations on the same image instance

To create high-quality datasets, it is necessary to have the same image annotated multiple times by different people (e.g. double or triple segmentations). At the moment there is no interface for this in MONAILabel.

My team could also contribute to this, but I am not sure what is the best way to implement this feature. Certainly some changes need to be made to the datastore, filter functions and plugins. Also some sort of user management is then necessary.

Currently, for multiple annotations, we use a workaround with multiple instances of MONAILabel sharing the same app. Each instance is then used by a different annotator. Later, the data must then be merged in a cumbersome way.

I would prefer that this functionality be integrated into MONAILabel in a bundled way. I am relatively sure that other teams will need this feature as well (@LilianaCaldeira, @dtruhn, @kbressem).

No files, tests, or entry points are named. Start by mapping how the datastore, filter functions, plugins, and annotation workflow represent an image instance and annotator; then clarify the user-management and data-model decisions with maintainers. Done should include an agreed design and integrated support for multiple annotations without separate MONAI Label instances.