Operator sidecar improvements

[azun]

Description

Sidecar support was added in #344 . This works fine but there are some pain points when using zookeeper-operator as a helm dependency:

• the sidecar manifest can't be easily reused between dependencies/other pods
• no way to use templates for the sidecars (must specify the full Container struct)
• repetitive across multiple installations: no way to overwrite values for a container as arrays are concatenated by helm

Importance

nice-to-have

Location

charts/zookeeper-operator/templates/

Suggestions for an improvement

The additionalSidecar property can be wrapped inside a named template. This generic template would use the data as it's defined in values.yaml. The parent chart would then be able to overwrite it and define container templates that can be reused and parameterized.