Extend Helm codegen to allow specifying passthrough environment variables

[shashankram]

Currently, all environment variables are statically defined in the operator spec. There is a desire to expose passthrough environment variables such that users can specify env vars to Deployments without needing to worry about the existing default env vars.

Due to Helm limitations, it's not possible to append or mutate an existing list value. See:
helm/helm#9325
helm/helm#6569

To overcome this, allow setting additional env vars using the extraEnvs property.