Revisit the different 'Values' types in the CLI install code

[ihcsim]

Following #3229, the CLI code and Helm library (pkg/charts) added 3 *Values types to capture CLI options and Helm variables. The purpose of this issue is to revisit these types and come up with a clean, reusable interface in the pkg/charts library, so that the CLI and webhook can consume these types.

cmd.installValues - this struct consists of fields that are interpolated in the control plane Helm chart when the user runs the linkerd install command. It consists of both configurable and non-configurable fields. They are primarily primitive types suitable for rendering the Helm templates.

charts.DefaultValues - this struct consists of fields that are mapped directly to the charts/linkerd2/values.yaml and charts/linkerd2/values-ha.yaml files. These default values replaced the previously hard-coded defaults found in the CLI code. Any options provided to the linkerd install command will override these values. They are primarily primitive types suitable for rendering the Helm templates.

charts.Values - this struct consists of fields that correspond to proxy and proxy-init properties that are interpolated in the partials chart. These fields are mainly relevant to proxy injection, depended by the pkg/inject library. This struct is made up of a number of custom types.

[ihcsim]

#3200 introduces a new charts.Chart struct in pkg/charts which can be made more useful for our purpose here. Essentially, we will like install/upgrade being able to tell pkg/charts to load the linkerd2 chart without having to worry about the details around templates, defaults, dependencies, file I/O etc. Conversely, inject should be able to tell pkg/charts to load the patch chart. All the embedded labels and annotations names, and function pointers in the existing cmd.installValues struct can also be moved into the shared library.