feat: [WIP] Allow OCI helm repositories to reuse the GCPServiceAccountKey (issue #19731)

[withnale]

This is a draft PR for discussion of #19731

It enables the GCPServiceAccountKey parameter to be used with helm OCI repositories, allowing for GCloud Workload Identity to be used.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 55.86%. Comparing base (2c8a574) to head (47e597c).
Report is 385 commits behind head on master.

Files with missing lines	Patch %	Lines
util/helm/cmd.go	0.00%	7 Missing ⚠️
pkg/apis/application/v1alpha1/repository_types.go	66.66%	4 Missing and 1 partial ⚠️
util/helm/client.go	28.57%	4 Missing and 1 partial ⚠️
util/git/client.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19744      +/-   ##
==========================================
+ Coverage   55.82%   55.86%   +0.03%     
==========================================
  Files         320      320              
  Lines       43981    44052      +71     
==========================================
+ Hits        24553    24610      +57     
- Misses      16869    16887      +18     
+ Partials     2559     2555       -4     

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