Add ability to specify pipeline function config with selector

[andsens]

Given this config

# Kptfile
apiVersion: kpt.dev/v1
kind: Kptfile
metadata:
  name: example
pipeline:
  mutators:
  - image: gcr.io/kpt-fn/apply-replacements:v0.1.1
    configPath: replace.yaml

# replace.yaml
apiVersion: fn.kpt.dev/v1alpha1
kind: ApplyReplacements
metadata:
  name: replace
  annotations:
    config.kubernetes.io/local-config: "true"
replacements:
- source:
    kind: ConfigMap
    name: some-config
    fieldPath: data.test
  targets:
  - select:
      kind: Deployment
      name: deployment.example
    fieldPaths:
    - metadata.name

I would love to see the ability to reference the function config like this instead:

[...]
  mutators:
  - image: gcr.io/kpt-fn/apply-replacements:v0.1.1
    configSelector:
      # No "kind" selector necessary since it's always "ApplyReplacements"
      name: replace

This change carries with it quite a few advantages:

• You are no longer limited to specifying one config per file. Configs that belong together semantically can reside in the same file.
• Preceeding pipeline functions can actually generate configs for subsequent functions since we don't reference a specific file that must exist from the get-go.
• It seems to be more in line with how the rest of kpt works, where filepaths are just metadata during processing.
• If preprocessing ever becomes a thing, a child package could change how a parent package behaves, allowing for more concise setups

The disadvantage I see is mostly the introduction of footguns:

• How a function behaves can be quite convoluted, if e.g. its config is changed multiple times before being applied.
• You no longer have the WYSIWYG guarantee for function configs: When debugging you'll always have to keep in mind that a bug might be caused by an inadvertent config change (though the configs are all their own kind, so it might not be that likely to happen unless using completely unguarded mutations)
• Finding a function config is not as direct. If you're unfamiliar with the package you'll spend time searching through it to find the referenced config.

[bgrant0607]

cc @natasha41575 @droot @yuwenma

[bgrant0607]

For cases where there is a specific KRM type that maps to a specific function, as with ApplyReplacements, my preferred solution is a function catalog mechanism, so that it doesn't need to be specified in the Kptfile at all.
https://github.com/kubernetes/enhancements/tree/master/keps/sig-cli/2906-kustomize-function-catalog

We just added a temporary hack to add the apply-replacements function to the Kptfile automatically in the UI, because I forget to do that half the time when running demos.
GoogleContainerTools/kpt-backstage-plugins#56

[bgrant0607]

Filed #3339 for the catalog-based mapping mechanism.

Are there other use cases where a selector would be desirable?

In general, I do think we need more flexibility with respect to where inputs come from. Other examples:
#3155 (comment)
#3280 (comment)

[bgrant0607]

A good point was raised in slack:
https://kubernetes.slack.com/archives/C0155NSPJSZ/p1656746811019709

It seems more appropriate to use KRM-based references and selectors than file paths.

We need to think about how this would interplay with other mechanisms, such as replay-based updates: #3329. And bulk package creation: #3347.

[droot]

This is a good candidate someone to contribute to kpt, will be more than happy to help someone guide how to get this done.