client,cmd/tailscale,ipn,tka,types: implement tka initialization flow

This PR implements the client-side of initializing network-lock with the
Coordination server.

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

tka/builder.go

@@ -12,6 +12,7 @@ import (
 
 // Types implementing Signer can sign update messages.
 type Signer interface {
+	// SignAUM returns signatures for the AUM encoded by the given AUMSigHash.
 	SignAUM(tkatype.AUMSigHash) ([]tkatype.Signature, error)
 }