Geo: Perform Git over HTTP requests as a custom action instead of Proxy requests

Before

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

After

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

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, authorization headers}
    S->>P: POST $PRIMARY/foo/bar.git/info/refs/?service=git-receive-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>C: return Git response from primary
    C-->>S: stream Git data to push
    S->>P: POST $PRIMARY/foo/bar.git/git-receive-pack
    P-->>S: HTTP/1.1 200 OK
    P-->>S: <response>
    S-->>-C: gitlab-shell receive-pack response

Problem

As we've agreed in Geo: Proxy Git push over SSH via Workhorse, not... (gitlab#387568 - closed) it makes sense to move the Git over HTTP requests to Primary to Gitlab Shell to avoid proxying large blobs of data via Gitlab Rails. Rails will provide authorization headers: Geo: Send authorization headers to Gitlab Shell... (gitlab#390101 - closed), Gitlab Shell needs to perform Git over HTTP request and send the response to user.

Proposal

Instead of performing proxy HTTP requests, let's perform Git over HTTPS request straight to Primary node. Gitlab Rails may return a feature flag, let's take it into account to be able to switch between behaviors.