feature: add the possibilty to add extra env var and secrets to geoserver pod

[AlexGacon]

Description

This PR gives the possibility to:

• inject additional environment variable into the GeoServer ConfigMap
• inject additional secrets into the GeoServer Secret when not using an external one
• inject additional environment variable definitions into the GeoServer container (can be used to rename items from an external secrets)

Type of Change

Please select the relevant option:

• Bug fix
• New feature
• Documentation update
• Refactoring
• Other (please describe)

Related Issue

If there is an existing issue related to this pull request, please reference it here.

closes #201

Checklist

Please ensure that your pull request meets the following requirements:

• The pull request is limited to one type (docs, feature, bug fix, etc.)
• The pull request is as small as possible. Consider opening multiple pull requests instead of one large one.
• The feature or bug fix has been discussed and documented in an issue beforehand.

Additional Notes

Inspired by https://github.com/kartoza/charts/tree/develop/charts

Thank you for creating this pull request