Feature request: join with key

Right now there are two Access Control options:

  • Private
    • Nodes must be authorized to become members
  • Public
    • Any node that knows the Network ID can become a member.

I would like to expand the private option with:
Nodes must be authorized or join with this super secret key to become members.

Rationale: we spawn robots that should join our network all the time. Upon spawning they register themselves to several services of which zerotier is one.
For us it is not possible to know how many or which robots will be spawned. Right now we have to manually verify and authorize them. It would be nice if we could hand them a key with which they can register themselves.

Alternative considerations: we could use the zerotier api with a key for this, but this would hand off way to much permissions to every single robot.