Improvements to kyaml fn framework

This commit creates a new version of the alpha configuration functions framework. Goals include:
- Make it easy to build multi-version APIs with the framework (not previously facilitated at all).
- Simplify the framework's APIs where redundant configuration options exist (leaving the most powerful, replacing others with helpers to maintain usability they provided).
- Make the Framework's APIs more consistent (e.g. between the various template types, usage of kio.Filter, field names)
- Decouple responsibilities (e.g. command creation, resource list processing, generation of templating functions).
- Make the framework even more powerfully pluggable (e.g. any kio.Filter can be a selector, and the selector the framework provides is itself a filter built from reusable abstractions).
- Improve documentation.
- Make container patches merge fields (notably list fields like `env`) correctly.

kyaml/fn/framework/example/doc.go

@@ -2,8 +2,13 @@
 // SPDX-License-Identifier: Apache-2.0
 
 // Package main contains an example using the the framework.
+// The example annotates all resources in the input with the value provided as a flag.
+//
+// To execute the function, run:
+//
+//   $ cat input/cm.yaml | go run ./main.go --value=foo
 //
 // To generate the Dockerfile for the function image run:
 //
-//   $ go run ./main.go .
+//   $ go run ./main.go gen ./
 package main

kyaml/fn/framework/example/input/cm.yaml

@@ -0,0 +1,9 @@
+# Copyright 2021 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: tester
+data:
+  some: data

kyaml/fn/framework/example/main.go

@@ -4,28 +4,30 @@
 package main
 
 import (
+	"fmt"
 	"os"
 
 	"sigs.k8s.io/kustomize/kyaml/fn/framework"
+	"sigs.k8s.io/kustomize/kyaml/fn/framework/command"
 	"sigs.k8s.io/kustomize/kyaml/yaml"
 )
 
 func main() {
 	var value string
-	resourceList := framework.ResourceList{}
-	cmd := framework.Command(&resourceList, func() error {
-		for i := range resourceList.Items {
+	fn := func(rl *framework.ResourceList) error {
+		for i := range rl.Items {
 			// set the annotation on each resource item
-			err := resourceList.Items[i].PipeE(yaml.SetAnnotation("value", value))
-			if err != nil {
+			if err := rl.Items[i].PipeE(yaml.SetAnnotation("value", value)); err != nil {
 				return err
 			}
 		}
 		return nil
-	})
+	}
+	cmd := command.Build(framework.ResourceListProcessorFunc(fn), command.StandaloneEnabled, false)
 	cmd.Flags().StringVar(&value, "value", "", "annotation value")
 
 	if err := cmd.Execute(); err != nil {
+		fmt.Println(err)
 		os.Exit(1)
 	}
 }