Introduce suggestions /v2 route

What does this MR do and why?

This MR adds a new route to support new prompt input as per the related issue.

POST /v2/completions

Attribute	Type	Required	Description	Example
prompt_version	int	no	The version of the prompt	1
repository_name	string	yes	The name of the repository (max_len: 255)	gitlab-shell
current_file.file_name	string	yes	The name of the current file (max_len: 255)	README.md
current_file.content_above_cursor	string	yes	The content above cursor (max_len: 100,000)	import numpy as np
current_file.content_below_cursor	string	yes	The content below cursor (max_len: 100,000)	def __main__:\n

Example request:

curl --silent \
     --request POST \
     --url http://codesuggestions.gitlab.com/v2/completions \
     --header 'authorization: Bearer glpat-.....' \
     --header 'Content-Type: application/json' \
     --data '{
       "prompt_version": 1,
       "repository_name": "awesome_project",
       "current_file": {
         "file_name": "main.py",
         "content_above_cursor": "\"\"\"\nImplement fastapi middleware to log all incoming requests\"\"\"\n",
         "content_below_cursor": "robin"
       }
     }'

Example response:

{
  "id": "id",
  "model": "codegen",
  "object": "text_completion",
  "created": 1678282416,
  "choices": [
    {
      "text": "\nimport logging\n\nfrom fastapi import FastAPI\nfrom fastapi.",
      "index": 0,
      "finish_reason": "length"
    }
  ]
}

• Additionally, this endpoint will:
  • strip whitespace on some fields.
  • validate maximum lengths.
  • return validation errors if validation fails.

How to set up and validate locally

1. Check out this branch
2. Build a local Docker image

   docker buildx build -t code-suggestions-api:dev .

3. Run the recently build docker image, map to port 5999 on localhost

   docker run --rm -p 5999:5000 -e BYPASSS_EXTERNAL_AUTH=true -it code-suggestions-api:dev

4. Use your favourite REST client or curl to hit the endpoint (hint: use jq to parse JSON response)

   curl --silent \
     --request POST \
     --url http://localhost:5999/v2/completions \
     --header 'Content-Type: application/json' \
     --data '{
       "prompt_version": 1,
       "repository_name": "awesome_project",
       "current_file": {
         "file_name": "main.py",
         "content_above_cursor": "\"\"\"\nImplement fastapi middleware to log all incoming requests\"\"\"\n",
         "content_below_cursor": "robin"
       }
     }' | jq .
   
   {
     "id": "id",
     "model": "codegen",
     "object": "text_completion",
     "created": 1678282416,
     "choices": [
       {
         "text": "\nimport logging\n\nfrom fastapi import FastAPI\nfrom fastapi.",
         "index": 0,
         "finish_reason": "length"
       }
     ]
   }