fix: allow existing zeebe exporters dir

[aabouzaid]

Which problem does the PR fix?

Fixes: #1011

What's in this PR?

Allow mounting Zeebe exporters dir as an extra volume:

zeebe:
...
  extraVolumes
  - name: exporters-zeebe
    emptyDir: {}
  extraVolumeMounts:
  - name: exporters-zeebe
    mountPath: /usr/local/zeebe/exporters

Checklist

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