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/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)
 	}
 }