Extend helm chart configuration for Templates

[eromanova]

This PR introduces the following changes in the ClusterTemplate, ServiceTemplate, and ProviderTemplate spec:

• New spec.helm.chart section of the HelmChartSpec type to support providing extended helm chart configuration. An example of the simplest allowed configuration:

spec:
  helm:
    chart:
      chart: chart-name

An example of the extended configuration with the source

spec:
  helm:
    chart:
      chart: chart-name
      version: 1.0.0
      interval: 15m0s
      sourceRef:
        kind: HelmRepository
        name: helm-repo

The version parameter is optional (the latest chart version will be used if unset). The sourceRef and interval parameters from the spec.helm.chart will be automatically populated with defaults if unset:

interval: 10m0s
sourceRef:
  kind: HelmRepository
  name: hmc-templates

where hmc-templates is the name of the default HelmRepository created by the hmc controller.

• The spec.helm.chartName and spec.helm.chartVersion parameters were removed as unnecessary.

Closes #615