Support defining dnsConfig for agent injector

**Is your feature request related to a problem? Please describe.**
I am running an EKS cluster with `cluster domain => cluster.local`. When I am trying to use vault-agent in namespaces other than Vault's namespace, vault-agent cannot resolve Vault's dns thus unable to inject secrets. (Vault Agent seem to only search `ec2.internal` instead of `*.cluster.local`)

I would like to have a way to define own `cluster domain` which appears in Injector's `resolv.conf` so I can continue to use "upstream" Helm chart.

**Describe the solution you'd like**
I would like a way to either:
- to define [dnsConfig](https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#pod-dns-config) for sidecar injector
or
- to add more parameters to `AGENT_INJECT_VAULT_ADDR`, something like:
{{ include "vault.scheme" . }}://{{ template "vault.fullname" . }}.{{ .Release.Namespace }}.svc`{{ if (ne .Values. injector.customDomain "") }}. {{ .Values. injector.customDomain }}{{ end }}`:{{ .Values.server.service.port }}

**Describe alternatives you've considered**
see above

**Additional context**
Add any other context or screenshots about the feature request here.

I am more than happy to create PRs if this feature request is being considered at all.

Start with the upstream Helm chart's agent injector configuration and the AGENT_INJECT_VAULT_ADDR template, then compare the requested dnsConfig behavior with Kubernetes pod DNS configuration. The work is done when the chart exposes a supported way to configure the injector's cluster domain or dnsConfig and the rendered injector pod receives the intended DNS settings.

Written by the indexing model from the issue text.