add link to attestestion in automated notification emails if one exists

[monperrus]

For packages with attestations, it would be great to have a link to the attestation directly in this email.

Example:

Hi monperrus!

A new version of the package crawler-user-agents (1.0.135) was published at 2024-05-03T05:05:53.600Z from GitHub Actions: https://github.com/monperrus/crawler-user-agents/actions/runs/8934354363/attempts/1 (triggered via a "workflow_run" event on git ref "refs/heads/master").
The shasum of this package is c8f24334b6b77843525c9a0f6fcf0b226fba34a9.

The provenance attestation is available at https://search.sigstore.dev/?logIndex=90728959

If you have questions or security concerns, you can contact us at https://www.npmjs.com/support.

Thanks,

The npm team.