Implement PEP-740: Attestation upload support

[lcarva]

PEP 740 describes changes to Python package repositories to handle digitally signed attestations. Implementing this ensures that pulp supports modern packaging security standards, allowing package maintainers to demonstrate the integrity and authenticity of their uploads.

Unlike other ecosystems, e.g. OCI and RPM, the software artifact is not signed, but instead, integrity is established via signed attestations.