Allow more flexibility in the code which converts CR to Helm values

[porridge]

As a developer of an operator, I would like to have more flexibility in the code which converts a custom resource object to the helm chartutil.Values. Currently such code needs to conform to the values.Mapper interface which simply maps chartutil.Values to another chartutil.Values, which is just map[string]interface{}, and does not allow returning errors.

Specifically the desired features are:

• ability to leverage Go type checking and IDE code inspection capabilities, by operating on typed API structs, rather than shapeless tree of string -> interface{}. This would be possible if the interface would accept an Unstructured which then can be converted into a typed struct.
• ability to reach out to the cluster to fetch other resources possibly referenced by the CR, and propagate context information as well as errors properly

/assign