Istio mtls support

Feature Request

Is your feature request related to a problem? Please describe:
I'm using istio 1.6.4 and would like to enable mtls. I've create a tidb cluster with the following:

kubectl create namespace tidb && \
kubectl label namespace tidb istio-injection=enabled && \
kubectl apply -n tidb -f 11-tidb/cluster.yaml

The cluster.yaml looks like the following:

apiVersion: pingcap.com/v1alpha1
kind: TidbCluster
metadata:
  name: cluster-1
spec:
  version: v4.0.1
  timezone: UTC
  pvReclaimPolicy: Retain
  discovery: {}
  pd:
    baseImage: registry.cn-beijing.aliyuncs.com/tidb/pd
    replicas: 1 
    storageClassName: high-performance
    requests:
      storage: "250Gi"
    config: {}
  tikv:
    baseImage: registry.cn-beijing.aliyuncs.com/tidb/tikv
    replicas: 1
    storageClassName: high-performance
    requests:
      storage: "250Gi"
    config: {}
  tidb:
    baseImage: registry.cn-beijing.aliyuncs.com/tidb/tidb
    replicas: 1
    service:
      type: ClusterIP
    config: {}

This works.
However once I enable mtls like the following:

kubectl apply -n tidb -f 11-tidb/peer-authentication.yaml

with this peer-authentication.yaml file:

apiVersion: "security.istio.io/v1beta1"
kind: "PeerAuthentication"
metadata:
  name: "default"
spec:
  mtls:
    mode: STRICT

I get this error when running sql statements against it:

ERROR 9001 (HY000): PD server timeout

It would be great if this could be supported.

Describe the feature you'd like:
Support for istio mtls

Describe alternatives you've considered:
I've considered using the native tls of tidb but would rather use istio mtls as I'm using it elsewhere and it theoretically should be more secure as it's checking the certificate from both sides.

Start by reproducing the cluster from the provided cluster.yaml and peer-authentication.yaml with Istio injection enabled, then inspect the SQL request path associated with the PD server timeout. Determine the operator changes required for STRICT mTLS support and verify that SQL statements work successfully after applying them.