Add async client support (AsyncAxmeClient)

[George-iam]

The SDK currently provides a synchronous client. Many Python frameworks (FastAPI, async agents) need an async interface.

What needs to happen:

• Add AsyncAxmeClient class using httpx.AsyncClient
• Mirror all methods from AxmeClient with async equivalents
• Add tests using pytest-asyncio

This is a significant contribution — async support is one of the most requested features.