feat: add direct starlette support for GraphEvent

[jamescalam]

This pull request introduces a new FAQ for a common user error, adds a method for encoding GraphEvent objects as JSON for compatibility with FastAPI and Starlette, and bumps the package version to 0.0.10rc1.

Documentation improvements:

• Added a new FAQ entry in docs/user-guide/faqs.md explaining the TypeError: object dict can't be used in 'await' expression, including the cause and solution for users defining nodes without async def.

Core library enhancements:

• Added an encode method to the GraphEvent class in graphai/callback.py to serialize events as JSON, improving integration with FastAPI and Starlette.

Project metadata:

• Updated the package version in pyproject.toml from 0.0.9 to 0.0.10rc1.