These validations were previously performed in the CLI frontend. There
are two motivations for moving these to the local backend:
1. The backend controls synchronization around the relevant state, so
only the backend can guarantee many of these validations.
2. Doing these validations in the back-end avoids the need to repeat
them across every frontend (e.g. the CLI and tsnet).
Updates tailscale/corp#27200
Signed-off-by: Harry Harpham <harry@tailscale.com>
ac74d28190