feature: (helm) support custom volumes and volumeMounts for epp

[delavet]

What type of PR is this?

/kind feature

What this PR does / why we need it:
please see #1944 for details
Which issue(s) this PR fixes:

Fixes #1944

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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This pull-request has been approved by: delavet
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[k8s-ci-robot]

Hi @delavet. Thanks for your PR.

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