Implement UCAN-based Authorization

[vthwang]

• Generate the server’s did:key and securely store its private key
• Adapt our current signature format for UCAN tokens
• Define and implement UCAN capabilities for Email: create, read, delete.
• Enable sub-directory app verification of UCAN signatures
  • Expose user public keys to another new GitHub repo (Not yet setup)
  • Allow this app to verify UCAN token without did-ucan app.