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/testdata/template-processor/container-patches/container.template.yaml

@@ -0,0 +1,6 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+env:
+- name: {{ .Spec.Key }}
+  value: {{ .Spec.Value }}

kyaml/fn/framework/testdata/template-processor/patches/patch.template.yaml

@@ -0,0 +1,5 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+spec:
+  replicas: {{ .Spec.Replicas }}

kyaml/fn/framework/testdata/template-processor/templates/deploy.template.yaml

@@ -0,0 +1,14 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: foo
+  namespace: bar
+spec:
+  template:
+    spec:
+      containers:
+      - name: foo
+        image: {{ .Image }}