feat(ISV-7320): add direct-sign-index-image task

[JakubDurkac]

REFERENCE PR ADDING A SIMILAR TASK (rh-direct-sign-image): #2301

Describe your changes

New Tekton task direct-sign-index-image for signing FBC index images via the container-signing pipeline. The task has similar functionality to old sign-index-image task, but it uses direct container signing instead of Radas/UMB solution. Also, it outsources all the existing core logic from bash to a new python util it invokes - direct_sign_index_image.py, passing all parameters as CLI arguments.

• Passes signing params (pyxis-server, pipeline, pipeline-image, requester, batch-max-size, etc.) as CLI arguments to Python
• Pyxis secret mounted at /etc/secrets, paths set via env vars
• Includes Tekton integration test using Python mock pattern
• Generated README from task YAML

Assisted-by: Claude Opus 4.6

Relevant Jira

ISV-7320

Checklist before requesting a review

• I have marked as draft or added do not merge label if there's a dependency PR
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• I read CONTRIBUTING.MD and commit formatting
• I have run the README.md generator script in .github/scripts/readme_generator.sh and verified the results using .github/scripts/check_readme.sh
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[mmalina]

It looks good in general, but not approving yet, until it's final. Also, a note on resources: From what I've seen, in many tasks, the memory had to be increased when switching to python. But maybe in this case it's ok - 1 GB is already a lot, so even if python adds some overhead, it shouldn't be that significant I'd say.

[JakubDurkac]

The release service util is merged (konflux-ci/release-service-utils#823). Waiting for release-service-utils fix PR of a related task to be merged. I assume we want to reference the image that comes out of it in this task before merging, same as task #2301 that's also waiting for that image.

[JakubDurkac]

The release-service-utils fix PR was merged, the image was updated in the reference task #2301 and I've updated the image here to match it. Should be ready for final review / approval @mmalina. Thanks.