refactor(drivers): extract drivers into hasql/driver/ package

[Pavkazzz]

Summary

Stack 4/7 — canonical driver implementations under hasql.driver.

• Add aiopg, asyncpg, psycopg3, aiopg.sa, asyncpgsa, and async SQLAlchemy drivers implementing PoolDriver.
• Keep old driver module paths as compatibility shims.
• Make AiopgSaDriver implement PoolDriver directly and preserve synchronous Engine.release() behavior.
• Retain the focused legacy SQLAlchemy DSN regression until the top-level module becomes a shim in refactor(compat): preserve legacy import paths #40.

Stack

Base: #38 (refactor/3-core)
Next: #40 (refactor/5-compatibility)

Verification

• Driver-focused regressions passed, including the aiopg.sa contract and release behavior.
• Ruff and mypy passed.
• Required GitHub CI: lint, mypy, and Python 3.10–3.14 PostgreSQL test matrix.