assorted: plumb tka initialization & network-lock key into tailscaled

- A network-lock key is generated if it doesn't already exist, and stored in the StateStore. The public component is communicated to control during registration.
 - If TKA state exists on the filesystem, a tailnet key authority is initialized (but nothing is done with it for now).

Signed-off-by: Tom DNetto <tom@tailscale.com>
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