Unmarshal Kubernetes objects from templates

✅ This MR is based on the template engine in !28 (merged). Do not merge it before !28 (merged) is merged.

What does this MR do?

This change introduces TemplateInventory. An inventory for Kubernetes objects that are deserialized from rendering YAML templates.

The TemplateInventory renders a named group of templates (using the Template engine). It accepts an optional Decoder for deserializing objects. When decoder is not provided it uses the default UniversalDecoder.

How to test

1. Create the template directory and sample YAML template files for Kubernetes resource:

   mkdir -p assets/templates/test-2/
   cat assets/templates/test-2/secret-1.yaml <<EOF
   apiVersion: v1
   kind: Secret
   metadata:
     name: {{ .Name }}-secret-1
     namespace: {{ .Namespace }}
   type: Opaque
   data:
     hello: d29ybGQK
   EOF

2. The following code snippet prints our Kubernetes resource details:

   package main
   
   import (
       "context"
       "fmt"
   
       corev1 "k8s.io/api/core/v1"
       metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
   
       "gitlab.com/gitlab-org/cloud-native/operator/pkg/framework/inventory"
   )
   
   func main() {
       i := &inventory.TemplateInventory{
           Name: "test-2",
           Data: map[string]any{
               "Name": "test-2",
               "Namespace": "tests",
           },
       }
   
       if err := i.Load(context.TODO()); err != nil {
           panic(err)
       }
   
       for _, obj := range i.Objects() {
           if sec, ok := obj.(*corev1.Secret); ok {
               fmt.Println(sec.Namespace, "/", sec.Name, ":", sec.Type)
               fmt.Println(string(sec.Data["hello"]))
           }
       }
   }

Author's Checklist

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Required

• Ensure a release milestone is set.
• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline on GitLab.com.
• When ready for review, MR is labeled workflowready for review per the MR workflow.

Expected

• Test plan indicating conditions for success has been posted and passes.
• Documentation is created or updated.
• Tests are added.

Related issues

Related to #19 (closed)