Fix DynamicIdentityFileCreds being incompatible with L7 Loadbalancers

[strideynet]

Closes #36229

changelog: Fixes refresh_identity = true preventing Access Plugins connecting to Teleport using TLS routing with a L7 LB.

[strideynet]

Tested locally with

package main

import (
	"context"
	glog "log"
	"os"
	"os/signal"

	"github.com/sirupsen/logrus"

	"github.com/gravitational/teleport/api/client"
	apidefaults "github.com/gravitational/teleport/api/defaults"
	"github.com/gravitational/trace"
)

func main() {
	err := run()
	glog.Fatal(err)
}

func run() error {
	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
	defer cancel()

	log := logrus.StandardLogger()
	logrus.SetLevel(logrus.DebugLevel)
	log.SetLevel(logrus.DebugLevel)

	creds, err := client.NewDynamicIdentityFileCreds("./admin_identity")
	if err != nil {
		return trace.Wrap(err, "loading creds")
	}
	teleportClient, err := client.New(ctx, client.Config{
		Addrs:       []string{"leaf.tele.ottr.sh:443"},
		Credentials: []client.Credentials{creds},
	})
	if err != nil {
		return trace.Wrap(err, "connecting")
	}
	defer teleportClient.Close()

	nodes, err := teleportClient.GetNodes(ctx, apidefaults.Namespace)
	if err != nil {
		return trace.Wrap(err, "get nodes")
	}
	log.Infof("Nodes: %d", len(nodes))
	return nil
}

And Caddy configured as an L7 LB.

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