enable appVersion as parameter

[piotrwielgolaski-tomtom]

it would be nice if plugin could pass app-version to helm
https://helm.sh/docs/helm/helm_package/

https://github.com/deviceinsight/helm-maven-plugin/blob/develop/src/main/kotlin/com/deviceinsight/helm/PackageMojo.kt#L100

in this way Chart.yaml placeholder could be avoided and source helm would be valid, which is helpful in case of skaffold integration.

It could be like with chartVersion that has default value as project version.