Message: AddObserver

The AddObserver message handler is used to add a new observer to the system. It first ensures that the caller is an admin policy group 2 account, which is a multi-sig. It then disables inbound messages and either

1. adds a new node account for the observer and resets the keygen for the TSS, OR

2. adds the observer address into all observer lists.

The functionality that it triggers depends on the AddNodeAccountOnly boolean flag in the message.