kowl volume values: from strings to arrays

[AlinAtAxual]

Extra volumes and extra volume mounts now accept arrays of objects instead of plain strings. Updated README.

This will enable the use of such volumes and volume mounts, which is currently not possible:

# Add additional volumes, e. g. for tls keys
extraVolumes:
  - name: "existing-secret-with-broker-certificates"
    secret:
      secretName: "local-kafka-brokers"
      optional: false

# Add additional volumes mounts, e. g. for tls keys
extraVolumeMounts:
  - name: "existing-secret-with-broker-certificates"
    mountPath: "/etc/tls/key-material"
    readOnly: true

[CLAassistant]

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[AlinAtAxual]

Hello, this is my first merge request. I don't know how to update this merge request to squash the commits. I don't want to merge 5 commits.