Allow enabling SSH certificates for enterprise users

Summary

Allow enabling SSH certificates for enterprise users.

Enabling this feature means that a group owner can generate an SSH certificate and use it for Git access to the group projects.

We need to provide this setting to make a user aware of all the risks of being impersonated by the group owner and ask them to explicitly agree to it.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/428236

Screenshot

Go to Profile > Account and Enable Allow authentication via SSH certificates under Enterprise user:

Only after that SSH certificates generated for the user can be used for authentication.