Feature: add the possibility to provide additional environment variables to GeoServer

[AlexGacon]

Feature Description

Since the chart allows to define the docker images for GeoServer, it will be nice to allow the possibility to provide additional environment variables to the deployment as well, in case of the custom made image requires it.

Proposed Solution

A usual solution in helm charts is to have at least a extraPodEnv entry but it can be extended to other typical items like a extraConfigMap (or existing config map), extraSecrets (or existing secrets) or additional mounts.

Additional Information

Any additional information or context that may be helpful in implementing the feature.

[AlexGacon]

For secrets and GeoServer, it is already supported if we maintain a secret outside the helm chart, since the whole secret is imported into the pod.