Add support for custom Helm charts from a repository

[craigwalton-dsit]

The initial custom Helm chart support will be limited to charts which are available in a local directory.

We want to make it possible to install a chart directly from a Helm repository. We can adapt the K8sSandboxEnvironmentConfig for this.

We'll need a chart in a registry for testing purposes e.g. https://github.com/helm/examples. Has a couple of issues: it won't conform to the chart requirements (no environments will be discovered) and it will fail if the release name generated (8 char random string) starts in a digit.