Is your feature request related to a problem? Please describe.
I want to configure some things in Vault by using environment variables that differ between instances. These are not secrets, so they don't belong in server.extraSecretEnvironmentVars, but I also don't want to hard-code them into values.yaml in server.extraEnvironmentVars, or have to put them on the command-line with --set each time I deploy.

Describe the solution you'd like
My preference would be to put the variables into a ConfigMap and have the Vault Helm chart pull from that. For instance, the Datadog Helm chart lets you do this:

envFrom:
- configMapRef
  name: config-map-name

where

apiVersion: v1
kind: ConfigMap
metadata:
  name: config-map-name
data:
  ENV_VAR_1: value_1
  ENV_VAR_2: value_2

Describe alternatives you've considered
Put the HCL that configures the same thing into a ConfigMap, mount it, and point Vault at that. This should work, but is also much more verbose. For example,

volumes:
  - name: extra-env
    configMap:
      name: extra-env
      items:
        - key: config
          path: config.hcl

volumeMounts:
  - mountPath: /vault/userconfig/extra-env
  - name: extra-env

extraArgs: "-c /vault/userconfig/extra-env/config.hcl"

and if there's any reason why merging that config file with the main one doesn't do what you want, then you're out of luck. For instance, what happens if you've got multiple storage stanzas, each with a different part of the config?