Add Google AI model to NL Git assistant endpoint

Igor Drozdovrequested to merge
id-move-git-command-to-vertex-ai into master

What does this MR do and why?

It is now possible to receive GoogleAI response by sending model=vertexai param to /ai/llm/git_command API endpoint. We need both at the moment to smoothly migrate Gitlab CLI to GoogleAI response. A feature flag for this change is not introduced because the change is non-breaking. The whole feature is already behind a feature flag (ai_git_command_ff).

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/414275+

Edited by Igor Drozdov

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