Add extensible transformation

[Liujingfang1]

This is to make kustomize libraries more flexible and extensible. Consider a kustomization, if an extra transformation is allowed, then user can specify their own transformations, such as replacing certain field to other strings, filling an docker image name.

[Liujingfang1]

Some thoughts: an extended transformer reads in []byte and returns []byte. The transformer itself can be written in other language.
For example, a transformer that patches multiple object based on matched labels need to

• filter the []byte to get resources to patch
• update the matched resources
• return the new []byte

[mxey]

When you say another language, do you mean running an external program that would need to have its own understanding of how to parse Kubernetes manifests?

I was just thinking, what about embedding Skylark? Just passing a data structure with the matching resources to a Skylark function and having the function return a modified version.

[mxey]

Actually, Jsonnet probably makes more sense than Skylark.

[monopole]

IMO, once #300 is in and #169 is closed, we've done enough to support for creation of simple variants on a base.

ksonnet, helm et al. already provide completely general transformation languages for folks that want that sort of thing.

[Liujingfang1]

With JSON patch support added and transformer configurations added, we can close this issue.