refactor: Add support for Code Suggestion in GitLab monolith

Current direction shifted towards serving code suggestions from GitLab monolith instead of Model Gateway.

This MR add support for new code suggestions API endpoint

To test this change manually in VSCode setting set feature flag codeSuggestionViaMonolith to true

{
    "gitlab.featureFlags": {
        "forceCodeSuggestionsViaMonolith": true, // force request to use monolith flow regardless where open project in VSCode has it repository hosted. Only for use to force GDK integration for local development
        "chat": true,
        "codeSuggestionsViaMonolith": true 
    },
}