Refactor that focuses on SRP improvements

See merge request gitlab-org/gitlab-shell!214

Release v8.1.0

See merge request gitlab-org/gitlab-shell!219

Add Git protocol v2

See merge request gitlab-org/gitlab-shell!217

* Use gl_id when we don't know if it's a key-X or user-X
* Use Actor.new_from(gl_id) which will figure out if it's a Key or User
* Use key_str when we're referring to key-X as key_id is confusing

Release GitLab Shell v8.0.0

See merge request gitlab-org/gitlab-shell!218

Add support for ssh certificates

Closes gitlab-ce#34572

See merge request gitlab-org/gitlab-shell!207

- Is possible that gl_id could be a 'user_id'
- Is a more accurate variable name

* Move gitaly, git-lfs and 2FA logic out from gitlab_shell.rb
* Streamline parsing of origin_cmd in GitlabShell
* Utilise proper HTTP status codes sent from the API
* Also support 200 OK with status of true/false (ideally get rid of this)
* Use HTTP status constants
* Use attr_reader definitions (var over @var)
* Rspec deprecation fixes

* Use constants for git commands
* private instead of protected

* Remove HTTP related requires
* Make protocol = GL_PROTOCOL the default

* Rename actor to key_id
* protected to private
* Move attr_reader defintions under private