Add support for new bundle specification in `cosign attest`

[codysoyland]

This PR adds support for the new Cosign Bundle Specification in cosign attest.

Related: https://github.com/sigstore/cosign/issues/3139

This is in draft for now pending:

• [ ] TSA timestamp payload modified per spec and added to bundle
• [ ] New annotations are added
• [ ] Support for fallback to referrers tag schema
• [ ] Tests

To test, run the following (replacing MY_PREDICATE and MY_IMAGE as needed):

go run ./cmd/cosign attest --predicate MY_PREDICATE --new-bundle-format MY_IMAGE

Summary

Release Note

Documentation