helm values.yaml updates do not work with lists

[aburan28]

Describe the bug
when using the image updater in git write back mode the image updater does not update values in the values.yaml file when the image.tag/name have lists.

an example

images:
  - name: blah
    image: quay.io/animage
    tag: v0.0.1

annotations

argocd-image-updater.argoproj.io/image-list: test=quay.io/animage
argocd-image-updater.argoproj.io/test.helm.image-name: images.0.image
argocd-image-updater.argoproj.io/test.helm.image-tag: images.0.tag

ultimately it picks up the image change but instead of updating it in place within the list of images it will add to the bottom of the values.yaml which ultimately does not trigger the image to be updated properly.

images[0]:
  - tag: v1.0.0 

The issue seems to be caused by the setHelmValue function which incorrectly assumes that the structure of the values is a map. https://github.com/argoproj-labs/argocd-image-updater/blob/master/pkg/argocd/update.go#L575

To Reproduce
Steps to reproduce the behavior:
See above.

Expected behavior
A clear and concise description of what you expected to happen.
an update is made to the values.yaml as shown below

images:
  - name: blah
    image: quay.io/animage
    tag: v1.0.0

Additional context
Add any other context about the problem here.

Version
0.15.2

Note feel free to assign this issue to me. I'm happy to open up a pr to fix this

[chengfang]

@aburan28 thanks for reporting the issue. Feel free work on it and PRs are welcome.

[chengfang]

In application annotation test.helm.image-name: images.0.image, I'm wondering if we should use images[0].image to access the element.