chore(override): boilerplate for override pkg and stub functions (#2612)

<!-- Provide summary of changes -->
Create `override` pkg and add some stub functions. Part of #2588.
<!-- Issue number, if available. E.g. "Fixes #31", "Addresses #42, 77" -->

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internal/pkg/template/override/override.go

@@ -0,0 +1,40 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package override renders the manifest override rules to the CloudFormation template.
+package override
+
+import "gopkg.in/yaml.v3"
+
+// CloudFormationTemplate overrides the given CloudFormation template by applying
+// the override rules.
+func CloudFormationTemplate(overrideRules []Rule, origTemp []byte) ([]byte, error) {
+	content, err := unmarshalCFNYaml(origTemp)
+	if err != nil {
+		return nil, err
+	}
+	ruleNodes, err := parseRules(overrideRules)
+	if err != nil {
+		return nil, err
+	}
+	if err := applyRulesToCFNTemplate(ruleNodes, content); err != nil {
+		return nil, err
+	}
+	output, err := marshalCFNYaml(content)
+	if err != nil {
+		return nil, err
+	}
+	return output, nil
+}
+
+func unmarshalCFNYaml(temp []byte) (*yaml.Node, error) {
+	return nil, nil
+}
+
+func marshalCFNYaml(content *yaml.Node) ([]byte, error) {
+	return nil, nil
+}
+
+func applyRulesToCFNTemplate(rules []*ruleNode, content *yaml.Node) error {
+	return nil
+}

internal/pkg/template/override/override_test.go

@@ -0,0 +1,33 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package override
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_CloudFormationTemplate(t *testing.T) {
+	testCases := map[string]struct {
+		wantedContent string
+		wantedError   error
+	}{}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			// GIVEN
+
+			// WHEN
+			actualContent, err := CloudFormationTemplate(nil, nil)
+
+			if tc.wantedError != nil {
+				require.EqualError(t, err, tc.wantedError.Error())
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, tc.wantedContent, actualContent)
+			}
+		})
+	}
+}

internal/pkg/template/override/rule.go

@@ -0,0 +1,38 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package override
+
+// Rule is the override rule override package uses.
+type Rule struct {
+	// pathSegment example: "ContainerDefinitions[0].Ulimits.HardLimit: 1024"
+	// pathSegment string
+	// value       interface{}
+}
+
+type ruleNode struct {
+	// next *ruleNode
+}
+
+func parseRules(rules []Rule) ([]*ruleNode, error) {
+	var ruleNodes []*ruleNode
+	for _, r := range rules {
+		if err := r.validate(); err != nil {
+			return nil, err
+		}
+		node, err := r.parse()
+		if err != nil {
+			return nil, err
+		}
+		ruleNodes = append(ruleNodes, node)
+	}
+	return ruleNodes, nil
+}
+
+func (r Rule) validate() error {
+	return nil
+}
+
+func (r Rule) parse() (*ruleNode, error) {
+	return nil, nil
+}