feat: add attestation support

[v1v]

Support https://github.com/actions/attest-build-provenance for the attestation.

[andyfeller]

@v1v : firstly, thank you for opening up this PR and building upon the exciting new capability in v2.49.0! ❤️

Everything looks straight forward with a few suggestions around documentation and wordage. 🎉 My primary concern is attest is the opposite side of the coin of using these attestations to verify that a build has gone through expected processes in being created; this change merely generates and uploads the attestations. 🙇

[andyfeller]

Want to put this on the @cli/package-security radar being new to contributing to the GitHub CLI; this action used by GitHub CLI extension authors to build and publish their extensions to gh-extension topic-based community.

This is the necessary first part before updating the GitHub CLI to check for extensions' attestations before installing or upgrading, which we haven't discussed yet.

[andyfeller]