Add generic code completions API endpoint
This MR adds a generic endpoint POST /v2/internal/code/completions
/v2/experimental/code/completions
that can be used to call Google Codey and GitLab Codegen model. At the AI-Assisted group, we need this endpoint to automate HumanEval benchmark and custom evaluation pipeline (aka prompt lib).
Please, check the body parameters in https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/model-dev/-/issues/17 to call a specific model.
Closes https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/model-dev/-/issues/17
This API endpoint can be reused for any other ongoing work following #161 (closed). However, if it conflicts with !194 (closed) and !206 (closed), we can move the target generic endpoint the experimental
folder.