Helm: chart fixes and improvements

[crabique]

Hello!

Thanks for the great exporter, I was very glad to see it already had a Helm chart.

This PR includes some fixes and improvements that make it easier to use without forking or applying Kustomize patches.

• fix broken ServiceMonitor
• add support for defining container extraVolumes and extraVolumeMounts:
  • required for reading a key off an externally managed secret (e.g. ExternalSecrets operator)
• add support for defining Deployment annotations:
  • required for Reloader controller integration to automatically restart the pods when a referenced Secret changes
• update the existing pod reload mechanism to also react to changes in the SSH key supplied in values.yaml
• bump chart version and exporter image (it is currently lagging two releases behind)
• fix various typos and chart lint problems
• update chart README with new features

Please let me know if you have any questions regarding any of the changes.

[crabique]

Thanks for the merge! I didn't expect this to be tagged 0.12.5 as is 😄

This means the exporter image in the chart defaults of 0.12.5 is still 0.12.4, is this something expected?