feature/featuretags: make QR codes modular (#18358)

QR codes are used by `tailscale up --qr` to provide an easy way to
open a web-page without transcribing a difficult URI. However, there’s
no need for this feature if the client will never be called
interactively. So this PR adds the `ts_omit_qrcodes` build tag.

Updates #18182

Signed-off-by: Simon Law <sfllaw@tailscale.com>

util/qrcodes/qrcodes_disabled.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_qrcodes
+
+package qrcodes
+
+import "io"
+
+func Fprintln(w io.Writer, format Format, s string) (n int, err error) {
+	panic("omitted")
+}
+
+func EncodePNG(s string, size int) ([]byte, error) {
+	panic("omitted")
+}