release: generate release catalog companions for build artifacts

[msarahan]

Why

RAPIDS currently creates a release by tagging repositories in dependency order and allowing those tags to trigger builds that publish directly to release channels. That makes the tag a prerequisite for producing the artifacts, forces downstream repositories to wait for upstream publication and CDN synchronization, and prevents us from validating the complete final-version artifact set before making it public.

The proposed release platform reverses that relationship: build the complete release candidate first, verify the exact bytes as a set, and only then promote those approved bytes and tag their source commits. For that to work, the shared build workflows need a release-candidate mode that:

• produces final-version artifacts without pushing a tag;
• keeps candidate artifacts out of public package channels;
• associates artifacts with the source and settings that produced it; and
• places artifacts and their evidence in private storage where the release platform can collect them by frozen release train.
• reuses existing build artifact from private storage

This PR supplies that integration for the standard Conda, wheel, and custom build workflows. It consumes the release catalog action developed in rapidsai/shared-actions#136. As a result, all of our projects that use standard conda and wheel workflows will be automatically enrolled and enabled for participation in the new release platform without any additional per-repo modification.

What changes

This adds a new build type, release-candidate, to the existing choices: [branch, nightly, pull-request]. The release-candidate build type selects the behaviors associated with the new release platform:

• Enables local tagging so that the packages that the builds that get produced appear "final" in the sense that it has no alpha tag in its version. For example, 26.10.00, not 26.10.00a123
• Disables any upload to public repos (anaconda.org / nightly channels)
• Skips test (tests are run later as a validation step)
• Uploads artifacts and extra identifying metadata to the private S3 bucket

See the shared-actions release catalog documentation for the companion layout, schemas, package parsing rules, evidence semantics, and S3 layout.

Tracks rapidsai/build-infra#381.