[AI Bundle][Agent] Add support for prompt as file

[OskarStark]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	--
License	MIT

Support files like:

{
  "role": "You are an expert code reviewer with deep knowledge of software engineering best practices, design patterns, and code quality.",
  "responsibilities": [
    "Review code for bugs and potential issues",
    "Suggest improvements for code quality and maintainability",
    "Identify security vulnerabilities",
    "Recommend better design patterns when appropriate",
    "Ensure code follows language-specific best practices"
  ],
  "tone": "constructive and educational",
  "approach": "Provide thorough but concise feedback with specific suggestions and examples when helpful"
}

but also TXT or MD files are supported.

AI Bundle config

ai:
    agent
        my-agent:
            model: 'gpt-4o-mini'
            prompt:
                file: '%kernel.project_dir%/prompts/code-reviewer.json'

Proof