Support prompt template with the prepended language identifier

This MR updates the prompt template by adding a language identifier to it. To get the language identifier, we follow the same approach that was used in preprocessing the data to fine-tune the codegen model in https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/model-dev/-/blob/main/data/df/preprocessing.py#L233.

Briefly, we infer the language id from the file name extension. Examples:

• f.py -> <python>
• f.min.js -> <js>

We do not prepend a language identifier if the filename extension was not recognized and is not in the predefined list.

How to run the server locally

1. Create virtualenv and init shell: poetry shell
2. Install dependencies: poetry install
3. Update the .env file in the root folder with the following variables:

   AUTH_BYPASS_EXTERNAL=true
   TRITON_HOST=localhost
   TRITON_PORT=8080
   FASTAPI_DOCS_URL=/docs
   FASTAPI_OPENAPI_URL=/openapi.json
   FASTAPI_API_PORT=5052

4. Get k8s credentials to access our k8s cluster: gcloud container clusters get-credentials ai-assist --zone us-central1-c --project unreview-poc-390200e5
5. Port-forward the triton server to access it locally: kubectl port-forward svc/model-k8s-triton -n fauxpilot 8080:8080 --address='0.0.0.0'
6. Start the model-gateway server locally: poetry run codesuggestions
7. Open http://0.0.0.0:5052/docs in your browser and run any requests to the codegen model

Ref: https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/22BLO