Add Groups::SshCertificate model to store group's CAs

This commit is the first step to supporting adding CAs to a group in order to be able to gain Git access to those groups using SSH certificates.

Related issues:

• Add support for git access control via SSH cert... (#408474 - closed)
• Add database table and models for group SSH cer... (#421537 - closed)

This MR extracts the model creation from this merge request: Draft: Support uploading CA files to a group (!126741 - closed)

In order to view how this MR fits into the whole functional flow of the feature, please visit:

• Draft: Support uploading CA files to a group (!126741 - closed) - an example of UI/UX for adding a CA to a group
• Support authentication using SSH Certificates (gitlab-shell!812 - merged) - Gitlab Shell merge request to allow users to authenticate using SSH certificates signed by the CA