Rename and add fields additional context rest api

What does this MR do and why?

Resolves #481748 (closed), related to !164919 (merged), and gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1302 (merged)

This MR makes the following adjustments to support changes to the additional_context REST api to align with the GraphQL API as decided here:

• renames name -> id
• renames type -> category
• adds metadata - a new field with JSON type

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

Use cURL or PostMan to send a request to the Chat endpoint with additional context and verify it doesn't error:

http://gdk.test:3000/api/v4/chat/completions

{
    "content": "How can I write a method to say hello to someone based on these snippets?",
    "with_clean_history": true,
    "additional_context": [
        {
        "id": "postman_snippet_1.rb",
        "category": "snippet",
        "content": "puts 'Hello, World!'",
        "metadata": {"random": "hash"}
        },
        {
        "id": "postman_snippet_2.rb",
        "category": "snippet",
        "content": "def say_name(name)\n  puts \"Hello, #{name}!\"\nend\n\nsay_name('Melissa')"
        }
    ]
}