Async client variant (httpx) for concurrent market-data fetches

[martinkersner]

Problem

The SDK is requests-based and fully synchronous. Pulling many symbols/exchanges concurrently (a common market-data pattern) forces threads or serial calls.

Proposal

Offer an async client mirroring the sync surface:

• httpx.AsyncClient transport behind an AsyncAPI/AsyncResource (parallels the Resource composition from refactor: share one Session across the client tree via composition (#2, #3) #137).
• Same client.cex.candle(...) tree, await-able.
• Share as much request-building logic (_endpoints registry, param split) as possible between sync/async to avoid divergence.

Scope / caveats

Largest effort of the current backlog. Worth confirming real demand before building. Adds httpx as a dependency (or an optional datamaxi[async] extra).