ipn/store: add support for stores to hook into a custom dialer

For stores like k8s secrets we need to dial out to the k8s API as though Tailscale
wasn't running. The issue currently only manifests when you try to use an exit node
while running inside a k8s cluster and are trying to use Kubernetes secrets as the
backing store.

This doesn't address cmd/containerboot, which I'll do in a follow up.

Updates #7695

Signed-off-by: Maisem Ali <maisem@tailscale.com>

kube/client.go

@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -90,6 +91,12 @@ func (c *Client) SetURL(url string) {
 	c.url = url
 }
 
+// SetDialer sets the dialer to use when establishing a connection
+// to the Kubernetes API server.
+func (c *Client) SetDialer(dialer func(ctx context.Context, network, addr string) (net.Conn, error)) {
+	c.client.Transport.(*http.Transport).DialContext = dialer
+}
+
 func (c *Client) expireToken() {
 	c.mu.Lock()
 	defer c.mu.Unlock()