feat: GA of Kustomize and K8s manifests support (#1172)

This is the GA version of the helm-x integration #673 developed last year.

You get all the following benefits without an extra helm plugin:

- Ability to add ad-hoc chart dependencies/aliases, without forking the chart (Fixes #876 )
- Ability to patch resulting K8s resources before installing the helm chart
- Ability to install a kustomization as a chart (Requires `kustomize` binary to be available in `$PATH`
- Ability to install a directory of K8s manifests as a chart
- etc.

docs/advanced-features.md

@@ -0,0 +1,91 @@
+## Advanced Features
+
+- [Import Configuration Parameters into Helmfile](#import-configuration-parameters-into-helmfile)
+
+### Import Configuration Parameters into Helmfile
+
+Helmfile integrates [vals]() to import configuration parameters from following backends:
+
+- AWS SSM Parameter Store
+- AWS SecretsManager
+- Vault
+- SOPS
+
+See [Vals "Suported Backends"](https://github.com/variantdev/vals#suported-backends) for the full list of available backends.
+
+This feature was implemented in https://github.com/roboll/helmfile/pull/906.
+If you're curious how it's designed and how it works, please consult the pull request.
+
+### Deploy Kustomizations with Helmfile
+
+You can deploy [kustomize](https://github.com/kubernetes-sigs/kustomize) "kustomization"s with Helmfile.
+
+Most of Kustomize operations that is usually done with `kustomize edit` can be done declaratively via Helm values.yaml files.
+
+Under the hood, Helmfile transforms the kustomization into a local chart in a temporary directory so that it can be `helm upgrade --install`ed.
+
+The transformation is done by generating (1)a temporary kustomization from various options and (2)temporary chart from the temporary kustomization.
+
+An example pseudo code for the transformation logic can be written as:
+
+```console
+$ TMPCHART=/tmp/sometmpdir
+$ mkdir -p ${TMPCHART}/templates
+$ somehow_generate_chart_yaml ${TMPCHART}/Chart.yaml
+
+$ TMPKUSTOMIZATION=/tmp/sometmpdir2
+$ somehow_generate_temp_kustomization_yaml ${TMPKUSTOMIZATION}/kustomization.yaml
+$ kustomize build ${TMPKUSTOMIZATION}/kustomization.yaml > ${TMPCHART}/templates/all.yaml 
+```
+
+Let's say you have a `helmfile.yaml` that looks like the below:
+
+```yaml
+releases:
+- name: myapp
+  chart: mykustomization
+  values:
+  - values.yaml
+```
+
+Helmfile firstly generates a temporary `kustomization.yaml` that looks like:
+
+```yaml
+bases:
+- $(ABS_PATH_TO_HELMFILE_YAML}/mykustomization
+```
+
+Followed by the below steps:
+
+- Running `kustomize edit set image $IMAGE` for every `$IMAGE` generated from your values.yaml 
+- Running `kustomize edit set nameprefix $NAMEPREFIX` with the nameprefix specified in your values.yaml
+- Running `kustomize edit set namesuffix $NAMESUFFIX` with the namesuffix specified in your values.yaml
+- Running `kustomize edit set namespace $NS` with the namespace specified in your values.yaml
+
+A `values.yaml` file for kustomization would look like the below:
+
+```yaml
+images:
+# kustomize edit set image mysql=eu.gcr.io/my-project/mysql@canary
+- name: mysql
+  newName: eu.gcr.io/my-project/mysql
+  newTag: canary
+# kustomize edit set image myapp=my-registry/my-app@sha256:24a0c4b4a4c0eb97a1aabb8e29f18e917d05abfe1b7a7c07857230879ce7d3d3
+- name: myapp
+  digest: sha256:24a0c4b4a4c0eb97a1aabb8e29f18e917d05abfe1b7a7c07857230879ce7d3d3
+  newName: my-registry/my-app
+
+# kustomize edit set nameprefix foo-
+namePrefix: foo-
+
+# kustomize edit set namesuffix -bar
+nameSuffix: -bar
+
+# kustomize edit set namespace myapp
+namespace: myapp
+```
+
+At this point, Helmfile can generate a complete kustomization from the base kustomization you specified in `releases[].chart` of your helmfile.yaml and `values.yaml`,
+which can be included in the temporary chart.
+
+After all, Helmfile just installs the temporary chart like standard charts, which allows you to manage everything with Helmfile regardless of each app is declared using a Helm chart or a kustomization.