Optionally decouple version from appVersion

[almereyda]

As a user of the update Chart action, I need to specify the version independent from the appVersion, in order to use semantic versioning for the chart, that might not follow an app's release cycle.

Currently, when changing the appVersion, we also set the version of the chart to the same value (--new-version). This might not be ideal in some cases. It could be nice if we could optionally

• set version explicitly through something like --chart-version
• set appVersion explicitly through something like --app-version

This way we achieve a more fine-grained control of an app's and its chart's release processes.