Add: Cosign Sign

[Ludy87]

Description

@M0NsTeRRR Can you check the correctness

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
• I have read the section Add New Translation Tags (for new translation tags only)

[M0NsTeRRR]

I would avoid using an external action to upload binaries. Gh CLI is already available inside GH runner, you can use something like (haven't tested it but should be close to that)
https://cli.github.com/manual/gh_release_upload

 - name: Upload artifact to release
   run: gh release upload ${{ github.ref }} ./build/libs/Stirling-PDF${{ matrix.file_suffix }}.* ./build/launch4j/Stirling-PDF-Server${{ matrix.file_suffix }}.*
   env:
     GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

[Ludy87]

@M0NsTeRRR I have the signature created in the cosign attest-blob

[M0NsTeRRR]

I think it makes sense to have at least a single job that verifies every artifact is properly signed using GitHub keyless signing (https://docs.sigstore.dev/cosign/verifying/attestation/).
Otherwise, except for my two other comments that were marked as resolved, it looks good to me! :)