Sign Sylva-Units pushed in the OCI registry

What does this MR do and why?

This issue address the signature of sylva-units pushed to the OCI registry if, and only if, the Gitlab variables COSIGN_PRIVATE_KEY is set.

Related reference(s)

• FluxCD Verify the integrity of OCI artifacts
• RFE

Test coverage

• Script build-sylva-units-artifacts.py with signature ran manually
• CI launched without signature to check that the MR does not break the actual behaviour

Closes #1109 (closed)