types/key: add ControlPrivate.Shared wrapper too

Follow-up to cfdb862673.

Change-Id: Iab610d761f1e6d88e8bcb584d9c02cafe48fc377
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/key/control.go

@@ -56,6 +56,11 @@ func (k ControlPrivate) SealTo(p MachinePublic, cleartext []byte) (ciphertext []
 	return k.mkey.SealTo(p, cleartext)
 }
 
+// SharedKey returns the precomputed Nacl box shared key between k and p.
+func (k ControlPrivate) SharedKey(p MachinePublic) MachinePrecomputedSharedKey {
+	return k.mkey.SharedKey(p)
+}
+
 // OpenFrom opens the NaCl box ciphertext, which must be a value
 // created by SealTo, and returns the inner cleartext if ciphertext is
 // a valid box from p to k.