Add Node.StableTailnetID for control to send the current tailnet's
stable ID on the self node. Expose it as CurrentTailnet.StableID in
LocalAPI status and `tailscale status --json`, and display it in
`tailscale whoami`.
Bump CurrentCapabilityVersion to 148.
Updates #14375
RELNOTE: Show the current tailnet's stable ID in status JSON and whoami.
Change-Id: I0525ff735de8113c8d124045a94e00c19a5a02e2
Signed-off-by: Andrew Dunham <andrew@tailscale.com>