feat: support additional context in code completions

What does this merge request do and why?

Adds new context prompt component to v3 code completions endpoint

Example Payload

{
  "prompt_version": 3,
  "prompt_components": [
    {
      "type": "code_editor_completion",
      "payload": {
        "file_name": "main.go",
        "language_identifier": "go",
        "content_above_cursor": "func parseEnvFile(filename string) (map[string]string, error) {",
        "content_below_cursor": "}",
        "model_provider": "vertex-ai",
        "stream": false,
        "model_name": "code-gecko@002"
      }
    },
    {
      "type": "code_context",
      "payload": {
        "content": "import os",
        "name": "test",
        "type": "function"
      }
    }
  ]
}

Resolves #476

How to set up and validate locally

1. Add context to the v3 code completions request as context prompt components.
2. In the request sent to code-gecko, notice how the context has been added before the prefix (but after the comment) up to the model window size

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Closes #476