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