Add ability to set extra environmental variables through downward API

[dezmodue]

This PR provides a way to set extra environmental variables through the downward API.
Why?
When managing multiple clusters often the difference between helm values boils down to the authPath which is set to a path including the cluster unique ID.
With the latest release of ArgoCD it is now possible to leverage the argocd.argoproj.io/tracking-id annotation to provide the cluster ID to every deployment.
Leveraging the downward API we can programmatically read the cluster ID from the argocd.argoproj.io/tracking-id annotation and set the authPath for all clusters programmatically without the need for a cluster specific value file.

Example:
Generate the injector deployment:

helm template --show-only templates/injector-deployment.yaml \
--set 'injector.annotations.argocd\.argoproj\.io/tracking-id=cluster-1234' \  
--set "injector.extraEnvironmentVarsFieldPath.CLUSTER_ID=metadata.annotations['argocd.argoproj.io/tracking-id']" \
--set 'injector.authPath=/auth/Kubernetes/$(CLUSTER_ID)' .

This will produce the yaml:

...
  template:
    metadata:
      labels:
        app.kubernetes.io/name: vault-agent-injector
        app.kubernetes.io/instance: release-name
        component: webhook
      annotations:
        argocd.argoproj.io/tracking-id: cluster-1234
...
          env:
            - name: "CLUSTER_ID"
              valueFrom:
                fieldRef:
                  fieldPath: metadata.annotations['argocd.argoproj.io/tracking-id']
...
            - name: AGENT_INJECT_VAULT_AUTH_PATH
              value: /auth/Kubernetes/$(CLUSTER_ID)

The resulting env in the running pod shows:

CLUSTER_ID=cluster-1234
AGENT_INJECT_VAULT_AUTH_PATH=/auth/Kubernetes/cluster-1234

The generated Env variables and configuration is valid for any cluster removing the need for cluster specific values

[hashicorp-cla-app]

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