cmd/k8s-operator,k8s-operator/api-proxy: move k8s proxy code to library (#15857)

The defaultEnv and defaultBool functions are copied over temporarily
to minimise diff. This lays the ground work for having both the operator
and the new k8s-proxy binary implement the API proxy

Updates #13358

Change-Id: Ieacc79af64df2f13b27a18135517bb31c80a5a02
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

k8s-operator/api-proxy/env.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !plan9
+
+package apiproxy
+
+import (
+	"os"
+
+	"tailscale.com/types/opt"
+)
+
+func defaultBool(envName string, defVal bool) bool {
+	vs := os.Getenv(envName)
+	if vs == "" {
+		return defVal
+	}
+	v, _ := opt.Bool(vs).Get()
+	return v
+}
+
+func defaultEnv(envName, defVal string) string {
+	v := os.Getenv(envName)
+	if v == "" {
+		return defVal
+	}
+	return v
+}