Allow calls to `WatchIPNBus` to be permformed by clients with
Readonly permissions. This brings it in line with the permissions
required for `Status`, which also exposes the similar information.
This allows clients to get realtime updates about the tailnet
in their own applications, without needing to actively poll the
`Status` endpoint.
Fixes https://github.com/tailscale/tailscale/issues/7797
Signed-off-by: Dominic Black <dom@encore.dev>
570cb018da