Define an SDK library specification for KRM function authoring

[monopole]

Two languages are supported to author KRM functions:

• typescript; https://googlecontainertools.github.io/kpt/guides/producer/functions/ts/

• Go; https://googlecontainertools.github.io/kpt/guides/producer/functions/golang/

but there's no easy way to compare these libraries to generally understand the capabilities offered, and see what is missing.

We need a doc on the kpt site, possibly simply a markdown matrix, describing what's offered, and linking to the relevant packages.

It might help (or it might be overkill) to establish a language agnostic api definition using openapi.

[monopole]

https://pkg.go.dev/sigs.k8s.io/kustomize/kyaml/yaml https://www.npmjs.com/package/kpt-functions

[phanimarupaka]

@Shell32-Natsu Please add sizing and other details to this issue.

[mikebz]

@mengqiy might this be something you pick up? I think you have done a lot of the analysis here already.