[BCR] Update pre- and post-submit to support attestations

Parent issue: https://github.com/bazelbuild/bazel-central-registry/issues/3829

1. Pre-submit checking should happen in bcr_validation.py in the BCR repo so that users can run the checks locally.
2. Post-submit checks and mirroring are part of [bcr_postsubmit.py](https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/bazel-central-registry/bcr_postsubmit.py)