proposal: Refactor build-extension-charts workflow in master to use scripts from upstream (dashboard)

[aalves08]

Summary

Fixes #12824

Occurred changes and/or fixed issues

• Refactor build-extension-charts workflow in master to use scripts from upstream (dashboard) instead of shell package

Technical notes summary

The reusable workflows build-extension-charts and build-extension-catalog make use of a couple of scripts, as per https://github.com/rancher/dashboard/blob/master/.github/workflows/build-extension-charts.yml#L80-L102, that are being pulled from the client's shell package.

The goal here is to update these workflows to that we use the versions of those scripts in each branch (for latest would be master branch) so that we can apply fixes without the need for the client to update shell

Testing should be already covered in https://github.com/rancher/dashboard/blob/master/.github/workflows/test-extension-workflows-master.yml + https://github.com/rancher/dashboard/blob/master/.github/workflows/test-extension-workflows-release-2.9.yml + https://github.com/rancher/dashboard/blob/master/.github/workflows/test-extension-workflows-release-2.8.yml

This is a working proposal, tested on Elemental repo

Workflow that ran: https://github.com/rancher/elemental-ui/blob/0cfbc152983d6f8631cac1c022a6f216570e7796/.github/workflows/build-extension-charts.yml

pipeline success: https://github.com/rancher/elemental-ui/actions/runs/12483392216/job/34839112211

Extension installed:

The goal is, if there are no impediments by analysing this PR, to apply these changes to the release-2.8 and release-2.9 branches, so that we can, afterwards, attack with full force some small issues that we've identified with the scripts.

Areas or cases that should be tested

Areas which could experience regressions

Screenshot/Video

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