Migrate DataPack parsing to backend

[mylibrar]

Is your feature request related to a problem? Please describe.
Stave is maintaining a manual DataPack parsing function in its frontend codebase, which is susceptible to any updates on the serialization schema in Forte. We already have the DataPack serialization/deserialization implemented in Forte, and it might be hard to maintain two DataPack parsing systems at the same time. A more reasonable solution is to migrate the parsing logics to backend and let Forte handle most of the encoding/decoding work of DataPack.

Describe the solution you'd like

• Utilize DataPack.from_string() and DataPack.to_string() for parsing and encoding.
• Certain backend API should be updated.
• Frontend logics should also be changed correspondingly.

Describe alternatives you've considered
We might also need to refactor some of the handlers in stave_viewer.py to ensure that StaveProcessor won't break.

Additional context

• It might be blocked by Parse user specified ontology file forte#692
• This updates should be thoroughly tested.