[CI] Attest build artifacts

[iamazeem]

Reference: https://github.blog/news-insights/product-news/introducing-artifact-attestations-now-in-public-beta/

[iamazeem]

Attestations: https://github.com/liquidaty/zsv/attestations

[iamazeem]

Verified locally and it's working fine!

Steps:

• Downloaded artifacts from the PR (#197).
• Unzipped to an artifacts directory.
• Verified with gh attestation verify <path-to-download-artifact> --repo liquidaty/zsv command.
• Verification succeeded!
• Tempered the include/zsv.h file in some archives.
• Reran the verification command.
• Verification failed!
• Restoring the include/zsv.h file also made the verification failed.

[iamazeem]

Added note in README with 0f9e186.