wgengine/magicsock: extract IMDS utilities into a standalone package (#18334)

Moves magicksock.cloudInfo into util/cloudinfo with minimal changes.

Updates #17796

Change-Id: I83f32473b9180074d5cdbf00fa31e5b3f579f189

Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>

util/cloudinfo/cloudinfo_nocloud.go

@@ -0,0 +1,26 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ios || android || js
+
+package cloudinfo
+
+import (
+	"context"
+	"net/netip"
+
+	"tailscale.com/types/logger"
+)
+
+// CloudInfo is not available in mobile and JS targets.
+type CloudInfo struct{}
+
+// New construct a no-op CloudInfo stub.
+func New(_ logger.Logf) *CloudInfo {
+	return &CloudInfo{}
+}
+
+// GetPublicIPs always returns nil slice and error.
+func (ci *CloudInfo) GetPublicIPs(_ context.Context) ([]netip.Addr, error) {
+	return nil, nil
+}