Implement support for module attestations

[fweikert]

Context: #2721

1. Implement optional attestation support: Attestations are checked and mirrored, but are still optional (regressions are disallowed, though).
2. Update documentation.
3. Implement UI support to show which module versions have attestations.
4. Widely advertise attestation support once the generator GitHub action is ready to be used.
5. [TBD] Enforce existence of attestations for all future module versions after a migration period.

Mirroring happens during post-submit -> bazelbuild/continuous-integration#2177.

[fweikert]

Note: There are PRs with multiple modules, e.g. https://github.com/bazelbuild/bazel-central-registry/pull/3798/files