Add getter and setter functions for Helm values

NOTE: This is part of refactoring that is listed in #72 (closed)

This MR adds two new functions to helm.Values for getting and setting values. GetValue and SetValue functions will be used for both #72 (closed) (esp. for using Chart values instead of patching) and #84 (closed) where specific values must be read from the CR.

The key difference between AddValue and SetValue is that the former uses Helm CLI syntax (hence string manipulation) for both key and value while the latter is more compatible with Go syntax. For example we can't set complex map structures with AddValue but it can be done with SetValue. On the other hand AddValue has a simpler and familiar usage for scalar or array values.