cmd/tailscale/cli: use optimistic concurrency control on SetServeConfig

This PR uses the etag/if-match pattern to ensure multiple calls
to SetServeConfig are synchronized. It currently errors out and
asks the user to retry but we can add occ retries as a follow up.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/serve.go

@@ -44,6 +44,12 @@ type ServeConfig struct {
 	// of either the client or the LocalBackend does not expose ports
 	// that users are not aware of.
 	Foreground map[string]*ServeConfig `json:",omitempty"`
+
+	// ETag is the checksum of the serve config that's populated
+	// by the LocalClient through the HTTP ETag header during a
+	// GetServeConfig request and is translated to an If-Match header
+	// during a SetServeConfig request.
+	ETag string `json:"-"`
 }
 
 // HostPort is an SNI name and port number, joined by a colon.