Add the ability to access files inside templates

[fernandezcuesta]

Description of your changes

Similarly as Helm template does through the .Files object, we might need to literally inject vanilla files into a template. For example, rather than including the terraform code into workspaces.tf.upbound.io spec.forProvider.module, we might do:

apiVersion: tf.upbound.io/v1beta1
kind: Workspace
spec:
  forProvider:
    source: Inline
    module: {{ readFile "/templates/terraform/workspace.tf" | quote }}
    # module: {{ readFiles "/templates/terraform/*.tf" "\n" | quote }} # alternatively

and keep files separatedly, easier to maintain.

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[fernandezcuesta]

Just to add more clarity on the usecase, the idea is to inject some files "as is". These files are copied in configMaps which are:

• created by kustomize

configMapGenerator:
- name: my-composition
  namespace: crossplane-system
  files:
  - terraform/variables.tf
  - terraform/main.tf
  - terraform/data.tf
generatorOptions:
  disableNameSuffixHash: true
  labels:
    part-of: function-go-templating
  annotations:
    k8s-sidecar-target-directory: terraform

• read and injected via DeploymentRuntimeConfig this way:

apiVersion: pkg.crossplane.io/v1beta1
kind: DeploymentRuntimeConfig
metadata:
  name: function-go-templating
  # DeploymentRuntimeConfig is not namespaced, but this is required by
  # kustomize to properly inject the generated ConfigMap
  namespace: crossplane-system
spec:
  deploymentTemplate:
    metadata:
      labels:
        kubernetes.io/part-of: crossplane
    spec:
      selector: {}
      template:
        spec:
          containers:
          - name: configmap-mounter
            env:
            # All configmaps labeled `part-of: function-go-templating` will be
            # mounted in `/templates`
            - name: LABEL
              value: part-of
            - name: LABEL_VALUE
              value: function-go-templating
            - name: FOLDER
              value: /templates
            image: kiwigrid/k8s-sidecar:1.25.4
            imagePullPolicy: IfNotPresent
            volumeMounts:
            - mountPath: /templates
              name: templates
          - name: package-runtime
            args:
            - --debug
            volumeMounts:
            - name: templates
              mountPath: /templates
          volumes:
          - name: templates
            emptyDir: {}
  serviceAccountTemplate:
    metadata:
      name: function-go-templating # required by rolebinding
---
kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: crossplane-system
  name: template-replication
rules:
- apiGroups: [""]
  resources: ["configmaps"]
  verbs: ["get", "watch", "list"]
---
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: template-replication
  namespace: crossplane-system
subjects:
- kind: ServiceAccount
  name: function-go-templating
  namespace: crossplane-system
roleRef:
  kind: Role
  name: template-replication
  apiGroup: rbac.authorization.k8s.io