Helm: Consider independent template/resource YAML files

[manusa]

Description

Part of #865

Might be fixed while tackling #2386

In order for users to be able to provide their own Charts and associated template files, it'd be necessary for JKube to be able to consider independent YAML files representing each resource (such as deployment.yaml, service.yaml, and so on).

These YAMLs might not follow strict YAML syntax since they might contain Helm templating expressions (invalid for YAML).

Current state

Currently, JKube considers only the generated kubernetes.yaml or openshift.yaml files that aggregates the resources generated by JKube during the k8s:resource phase.

If this file doesn't exist, the icon retrieval makes the whole execution fail. Fixed by #2386

The following lines log a warning in case the generic file is missing, but don't fail the execution.

https://github.com/eclipse/jkube/blob/5da6068d7756784aef9569568ee80f98da97e296/kubernetes-maven-plugin/plugin/src/main/java/org/eclipse/jkube/maven/plugin/mojo/build/HelmMojo.java#L59-L61

https://github.com/eclipse/jkube/blob/5da6068d7756784aef9569568ee80f98da97e296/gradle-plugin/kubernetes/src/main/java/org/eclipse/jkube/gradle/plugin/task/KubernetesHelmTask.java#L36-L38

Solutions

• Add a HelmConfig parameter to provide the source of an existent chart+templates directory that will be copied (unprocessed) to the chart output location.
• Enhance the additionalFiles handling to fulfill this purpose.