feat: add sync and async client APIs

[ijelacicbot]

Summary

• Add AsyncClient with the same Judge0 operations as the sync Client.
• Add async flavor clients for Judge0 Cloud, RapidAPI, and AllThingsDev.
• Allow injecting httpx.Client / httpx.AsyncClient for tests and custom transports.

Same work as #51, opened from judge0/judge0-python so Actions can receive repository secrets. Rebased onto current master (CHANGELOG only).

Motivation

Closes #50

The SDK only had a blocking Client on httpx.Client. Async applications had to wrap calls in a thread pool.

Changes

• Client accepts optional http_client= and exposes close() / context manager.
• New AsyncClient with awaited methods and aclose() / async context manager.
• Flavor counterparts: AsyncJudge0CloudCE, AsyncRapidJudge0CE, AsyncATDJudge0CE, Extra CE variants, and base classes.
• Offline unit tests use HTTPX MockTransport.
• Docs and changelog updated. HTTPX2 was not adopted; this stays on httpx.

Test Plan

• uv run pytest tests/test_async_clients.py
• uv run ruff check src tests
• uv run pyright src tests
• Live client tests on this same-repo PR (JUDGE0_* secrets)

Notes for Reviewers

Existing async_execute / async_run still mean "do not wait for results". They are not asyncio coroutines. This PR adds real asyncio clients.

Fork PR: #51