feat (at_policy): Acting on behalf of another atSign policy model

[XavierChanth]

Ability to cryptographically verify an acting on behalf of scheme.

Considerations:

• Signing
• Temporarily allow the action (expiry)
• Capability to enforce that the client must pre-register with the policy service before the party acting on its behalf is allowed to do so
• List of acting on behalf of chain

Example of acting on behalf of chain:

flowchart LR
  A[A] --> B[B]
  B --> C[C]
  C --> D[D]

Loading

Model 1 - most capable:

A->B is a normal policy request.
B->C is B acting on behalf of A.
C->D is C acting on behalf of B, acting on behalf of A.

Model 2 - more simple:

A->B is a normal policy request.
B->C is B acting on behalf of A.
C->D is C acting on behalf of A.