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.

[koladefaj]

Hi @George-iam,

I'd like to work on adding async client support with AsyncAxmeClient (#37).

I have strong production experience with FastAPI + async patterns from my AI Document Intelligence Platform (which uses Celery + async workflows) and E-Pharmacy backend. I also recently contributed to sandhi_ai, where I migrated scheduling from APScheduler to Celery Beat (PR #79).

I'm happy to:

• Create AsyncAxmeClient using httpx.AsyncClient
• Mirror all methods from the existing AxmeClient as async versions
• Add comprehensive tests using pytest-asyncio

This will make the SDK much more useful for FastAPI and async agent workflows.

Let me know if I can start

Thanks