feat: allow write-back to actual kustomization files

[iamnoah]

fixes #199

I don't love the new type and annotations, this feels like a sub-option of the git method, but I don't have a great idea for better options.

[codecov]

Codecov Report

Merging #200 (89f3012) into master (b47f7e9) will increase coverage by 0.06%.
The diff coverage is 58.16%.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   66.71%   66.77%   +0.06%     
==========================================
  Files          20       20              
  Lines        1517     1574      +57     
==========================================
+ Hits         1012     1051      +39     
- Misses        412      424      +12     
- Partials       93       99       +6     

Impacted Files	Coverage Δ
pkg/argocd/git.go	60.95% <51.94%> (+1.15%)	⬆️
pkg/argocd/update.go	70.56% <80.95%> (+0.83%)	⬆️

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[jannfis]

Hey, how about this. We do have write-back-method annotation for specifying the method of writing back (e.g. argocd and git currently), we could have an additional write-back-target annotation to specify the target of the write-back.

That new annotation could have values override[:<app-source.yaml>] (default, uses the .app-source.yaml) or kustomize[:<kustomization.yaml>], each with an optional parameter (after a colon) to specify which file to actually override - e.g. some people don't use kustomization.yaml but Kustomization.yaml or even plain Kustomization. And maybe it can also be used to specify some path within the repository to use.

Oh, and never mind about implementing the override option with this PR, we can introduce it along with parameters later on. We should just make sure that the default behavior stays the same after this change (e.g. writes an app source override if write-back-target is not given).

[jannfis]

Regarding the spell checking of the docs, please add the "unknown" words it complains about (if they are not typos after all) to https://github.com/argoproj-labs/argocd-image-updater/blob/master/.github/actions/spelling/allow.txt

[jannfis]

Hey @iamnoah, can you please take a look at the complaints of the linter (as described here)

[jannfis]

LGTM, thanks @iamnoah