Add extra EnvVars, VolumeMounts, Volumes and InitContainers to the STS

[schferbe]

What this PR does / why we need it:

This PR adds extraEnvVars, extraVolumeMounts, extraVolumes and extraInitContainers to the values.yaml and StatefulSet. The rendering of the values is performed with bitnami/common. This allows users of the Helm chart to better customize their CouchDB deployment.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• no issue has been opened to my knowledge

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.

• [x] Chart Version bumped
• [x] e2e tests pass
• [x] Variables are documented in the README.md
• [x] NEWS.md updated