Authentication, signing, key management; consolidate with existing nanopub and dokieli libraries

[VirginiaBalseiro]

Tracking issue. The actual work belongs in the dokieli repo. Corresponding issue(s) to be created there. This issue exists only to track that dependency from our side.

• File issue(s) on dokieli repo
• Link them here once created

[VirginiaBalseiro]

dokieli/dokieli#604