feat: add environment config for monitoring agent container

[ndbaker1]

Issue #, if available:

Description of changes:

Add environment variable options to monitoring agent helm chart values.

Testing Done:

helm template eks-node-monitoring-agent ./charts/eks-node-monitoring-agent --namespace kube-system --debug --set nodeAgent.env.foo=bar

...
      serviceAccountName: eks-node-monitoring-agent
      hostNetwork: true
      hostPID: true
      containers:
        - name: eks-node-monitoring-agent
          image: 602401143452.dkr.ecr.us-west-2.amazonaws.com/eks/eks-node-monitoring-agent:v1.2.0-eksbuild.1
          imagePullPolicy: IfNotPresent
          args:
            - --probe-address=:8002
            - --metrics-address=:8003
          env:
            - name: HOST_ROOT
              value: /host
            - name: MY_NODE_NAME
              valueFrom:
                fieldRef:
                  fieldPath: spec.nodeName
            - name: foo
              value: "bar"
          livenessProbe:
            httpGet:
              path: /healthz
              port: 8002
          resources:
            limits:
              cpu: 250m
              memory: 100Mi
            requests:
              cpu: 10m
              memory: 30Mi
          securityContext:
...

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.