Need standardized API for TEA Artifact downloading

As discussed on the Ecma meeting on 2026-05-06, we need standardized API to download TEA Artifacts (i.e., actual SBOM content).

Currently, we have `url` field under `formats` that allows to specify external URL to download TEA Artifact. However, this should be considered as a fallback option only - under the assumption that such URL is either publicly available or is using same authn/z as the TEA server itself.