Expose async client

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe.

I am running an async codebase, and alpaca-py is blocking

### Describe the solution you'd like.

Either expose an async client (httpx is great), or add to the documentation a discussion of the concept (for example, workarounds, thread-safety, etc).

### Describe an alternate solution.

_No response_

### Anything else? (Additional Context)

Y'all are doing great work. Thanks!