Make Certificate messages beacon field optional

[jpraynaud]

Why

While implementing the issue #1804, we have made the usage of the beacon field in the pending certificate optional. We want to do this operation on the Certificate messages for completeness.

What

Make the beacon field optional in the Certificate messages.

How

• Make beacon field optional in CertificateMessage of Open API (make a test with a stable client)
• Make beacon field optional in CertificateListItemMessage of Open API (make a test with a stable client)