Add internal API endpoints for authorized SSH certificates

Overview

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

Proposal

The /authorized_certs API endpoint should be added in order to respond which namespace a particular user has access to via specified certificate.

The /allowed endpoint should accept namespace_path param that contains the namespace a user has access to. It should be verified whether the namespace_path is an ancestor of the passed project

• PoC of the whole functionality: Draft: Support uploading CA files to a group (!126741 - closed). The internal /authorized_certs and /allowed logic modifications can be extracted from there.
• Gitlab Shell MR that uses the endpoint: Support authentication using SSH Certificates (gitlab-shell!812 - merged)