Add database table and models for group SSH certificates

Overview

More information about the motivation in epic: &11192 (closed)

Proposal

A Groups::SshCertificate model should be added to store information about an SSH certificate added to a group:

• PoC of the whole functionality: Draft: Support uploading CA files to a group (!126741 - closed)
• Extracted MR to add the table/models: Add Groups::SshCertificate model to store group... (!127856 - merged)