Allow maps in environment variables in helm charts

[pyalex]

What this PR does / why we need it:
Currently it's not possible to use more complicated expressions (than string or int) as environment variable in kubernetes Deployment object via helm charts. Map structure could be useful to provide kube metadata as parameter to feast services.

env:
- name: MY_POD_NAMESPACE
  valueFrom:
    fieldRef:
      fieldPath: metadata.namespace

Now this can be achieved with
--set envOverrides.MY_POD_NAMESPACE.fieldRef.fieldPath=metadata.namespace

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

[pyalex]

/test test-end-to-end-redis-cluster

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: khorshuheng, pyalex

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[khorshuheng]

/lgtm