Geo: Move HTTP to SSH conversion from Rails to Gitlab Shell

Current architecture

Currently, when a user performs a Git over SSH request to secondary node, Gitlab Shell performs requests to secondary Workhorse & Rails which performs a Git over HTTP request to primary.

sequenceDiagram
    participant C as Git on client
    participant S as GitLab Shell
    participant I as Workhorse & Rails
    participant P as Workhorse & Rails

    Note left of C: git fetch
    Note over S,I: Secondary site
    Note over P: Primary site
    C->>+S: ssh git receive-pack request
    S->>I: SSH key validation (api/v4/internal/authorized_keys?key=..)
    I-->>S: HTTP/1.1 300 (custom action status) with {endpoint, msg, primary_repo}
    S->>I: POST /api/v4/geo/proxy_git_ssh/info_refs_receive_pack
    I->>P: POST $PRIMARY/foo/bar.git/info/refs/?service=git-receive-pack
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to push
    S->>I: POST /api/v4/geo/proxy_git_ssh/receive_pack
    I->>P: POST $PRIMARY/foo/bar.git/git-receive-pack
    P-->>I: HTTP/1.1 200 OK
    I-->>S: <response>
    S-->>-C: gitlab-shell receive-pack response

Proposal

Gitlab Rails has to perform some SSH to HTTP "translation" to make it work. The translation can be performed by scanning the payload line by line, that's why it makes sense to move it to Gitlab Shell, where we already have pktline scanner.

It's the first step to fixing Geo: Fix timeouts when pushing via SSH to secon... (&8819 - closed)