Kpt functions eventually should get their configuration from OpenAPI

[mengqiy]

Kpt fn may different configurations depending what their jobs are.
e.g. set-namespace function needs to know what is the path to the namespace field and what resources are namespaced resources.
e.g. set-label function needs to know what is the paths to the labels and selectors fields.

2 things need to be solved before we get there:

• Ability to configure the OpenAPI document used by kpt (design in progress)
• OpenAPI document capturing references information (e.g. namespaces)

This was originally discussed in GoogleContainerTools/kpt-functions-catalog#123 (comment)

[natasha41575]

will be part of #1521