[FEATURE] Implement Per-Arch Builds

[adambkaplan]

Overview

Following up on #2075, enhance the BuildRun controller to do the following for multiarch builds:

• Create PipelineRun for orchestrating the build.
• Execute TaskRun per architecture that pushes container image.
• Report image references and digests for individual architecture images.

Acceptance Criteria

• BuildRun controller creates PipelineRun for multiarch build
• Output images per architecture are pushed to container registry with appropriate tags
• BuildRun object status includes per-architecture image reference and digest
• Error handling for partially failed builds
• Unit tests pass with >80% coverage for new code
• Integration or e2e tests verify behavior

Dependencies

• Blocked by: [FEATURE] Implement multi-arch build API and validations #2075
• Blocks: [FEATURE] Implement image index for multi-arch images #2076

Technical Notes

• Per-arch builds need their own tags to ensure the destination container registry does not delete the image (this is a common GC practice with public container registries)
• Default behavior should maintain backward compatibility (single-arch builds)
• Testing: do we need to ask the CNCF for test clusters??