Refactor provider.py

[s-heppner]

In order to prepare for Registry and Discovery, we need to refactor provider.py.

• Make AbstractObjectProvider and AbstractObjectStore really abstract (e.g. independent of Identifiable)
  • Adapt DictObjectStore and SetObjectStore to the refactored functions, providing aliases with deprecation warning.
• Rename DictObjectStore and SetObjectStore to DictIdentifiableStore and SetIdentifiableStore and provide alias to old name with deprecation warning.

This way, we ensure backward compatibility in this core functionality of our SDK.