[Improvement] support setting the cluster username and password when create the Elasticsearch cluster

[shuoshadow]

Using environment variables to set it would be convenient, or other more secure methods.

Here is an example:

apiVersion: v1
kind: Secret
metadata:
  name: elastic-credentials
  namespace: kb-cloud
type: Opaque
stringData:
  username: elastic
  password: vYxdfazdfIJnnKas

apiVersion: apps.kubeblocks.io/v1alpha1
kind: Cluster
metadata:
  annotations:
    kubeblocks.io/extra-env: '{"mdit-roles":"master,data,ingest,transform","mode":"multi-node"}'
  labels:
    app.kubernetes.io/instance: dev-es
    app.kubernetes.io/version: 7.10.1
    helm.sh/chart: elasticsearch-cluster-0.9.0
  name: dev-es
  namespace: kb-cloud
spec:
  tolerations:
    - key: "node-type"
      operator: "Equal"
      value: "mid"
      effect: "NoSchedule"
  affinity:
    podAntiAffinity: Required
    tenancy: SharedNode
    #topologyKeys:
    #- kubernetes.io/hostname
    nodeLabels:
      node-type: "mid"
  componentSpecs:
    - componentDef: elasticsearch-7
      disableExporter: false
      env:
        - name: ELASTIC_USERNAME
          valueFrom:
            secretKeyRef:
              name: elastic-credentials
              key: username
        - name: ELASTIC_PASSWORD
          valueFrom:
            secretKeyRef:
              name: elastic-credentials
              key: password
      name: mdit
      replicas: 3
      resources:
        limits:
          cpu: "4"
          memory: 8Gi
        requests:
          cpu: "4"
          memory: 8Gi
      serviceAccountName: null
      serviceVersion: 7.10.1
      services: null
      systemAccounts:
      - name: elastic
      volumeClaimTemplates:
      - name: data
        spec:
          accessModes:
          - ReadWriteOnce
          resources:
            requests:
              storage: 500Gi
          storageClassName: openebs-hostpath
  terminationPolicy: Delete