Draft: Support uploading CA files to a group

What does this MR do and why?

Related issue: Add support for git access control via SSH cert... (#408474 - closed)

Gitlab Shell: Support authentication using SSH Certificates (gitlab-shell!812 - merged)

Screenshot

This MR introduces this UI for the convenience of adding a CA file and verifying the PoC of Add support for git access control via SSH cert... (#408474 - closed). UX Design hasn't been discussed yet.

Group Settings > General > Certificate Authority

Functional flow:

The functional workflow is the following:

• An admin adds CA.pub to a namespace
• A user tries authenticating using a certificate signed by the CA
• Gitlab Shell sends the fingerprint of the CA to /authorized_certs and returns the namespace, which is remembered on Gitlab Shell during an SSH connection. The CA is unique for an instance (unique index for the fingerprint). It means that it's not necessary to integrate a namespace into the certificate, a certificate and a namespace have 1 -> 1 relationship.
• The namespace is sent every time /allowed is called. It means that we send project fullpath and the namespace in which a user is authenticated. If the project doesn't have the namespace in the list of its ancestors, the action is not allowed.

sequenceDiagram
    User->>+Gitlab Shell: Auth using SSH Certificate
    Gitlab Shell->>+Gitlab Rails: /authorized_certs?key=signing-key-of-certificate&user_identity=username-or-primary-email
    Gitlab Rails-->>-Gitlab Shell: responds with the namespace that configures the CA and username of the user
    Gitlab Shell-->>User: Authenticated successfully
    User->>+Gitlab Shell: Git command to a specific project
    Gitlab Shell->>+Gitlab Rails: /allowed [allowed_namespace=namespace]
    Gitlab Rails-->>-Gitlab Shell: responds that the project belongs to this namespace or its ancestor
    Gitlab Shell-->>User: success