Add support for Plugin type applications with argocd write-back

[rpg600]

This PR introduces support for updating container images in Plugin type applications with the imperative argocd write-back method.

Key Features:

• Environment Variable Modification: Instead of adding parameters to the helm source of the application manifest, this PR modifies the HELM_ARGS environment variable within the plugin source.

• Integration with argocd-vault-plugin: The HELM_ARGS env variable is primarily used by argocd-vault-plugin to pass arguments to the helm template command before it generates the final manifest with vault values. For more details, visit https://argocd-vault-plugin.readthedocs.io/en/stable/usage/#with-additional-helm-arguments

Example of my use case:

Annotations:

argocd-image-updater.argoproj.io/image-list: php=registry.myapp.cloud/myapp/php:dev, nginx=registry.myapp.cloud/myapp/nginx:dev
argocd-image-updater.argoproj.io/php.helm.image-tag: php.image.tag
argocd-image-updater.argoproj.io/nginx.helm.image-tag: nginx.image.tag
argocd-image-updater.argoproj.io/update-strategy: digest

• Every time a commit is pushed to the dev branch, images are built with the dev tag in the CI pipeline.

• The dev tag is mutable, which is why the digest strategy is used.

• php.image.tag and nginx.image.tag are keys inside the values.yaml file that need to be overridden if a new digest of the images tag exists.

• With auto-sync enabled, the application is deployed automatically.

Initial manifest:

source:
  plugin:
    env:
      - name: HELM_ARGS
        value: >-
          -f ../../dev/charts/myapp/values.yaml

Updated manifest after an image update:

source:
  plugin:
    env:
      - name: HELM_ARGS
        value: >-
          -f ../../dev/charts/myapp/values.yaml 
          --set php.image.tag=dev@sha256:fb4b84725901d2c7214e080f2314e55dbc9c2cf2b7a9845c288c3014c7d614ab
          --set nginx.image.tag=dev@sha256:1e6a0957539cfb89a691ee721d4cd23ec5f4a9870c674fe4e8a50896d9ba386e

• All previously defined arguments are preserved.
• If a --set argument is already defined, it will be modified; otherwise, it will be added.

How to try it:

You can try out this feature by applying the following command:

kubectl apply -n argocd -f https://raw.githubusercontent.com/rpg600/argocd-image-updater/plugin-type-support-wmanifest/manifests/install.yaml

It's based on v0.13.1

Notes:

This PR was primarily focused on my specific use case and may not cover all possible scenarios, but I wanted to share it for broader use and feedback. I have not tested the Git write-back method. However, the related PR #637 appears to add this support.

It fixes #417