Add sync wrapper, examples, and tests

[gregsvo]

• Adds postmark.sync — a synchronous wrapper around the async clients, so the SDK can be used without async/await
• Reorganizes all examples into examples/async/ and examples/sync/
• 31 additional tests: tests/test_sync_client.py

The library is async-first, but users in scripts, Flask apps, and Jupyter notebooks often want a simpler synchronous API. The new postmark.sync module provides this without duplicating any business logic.

How it works: A single daemon thread with a persistent asyncio event loop is started on first import. Each sync call submits a coroutine to that loop via asyncio.run_coroutine_threadsafe() and blocks the calling thread until it completes. The underlying httpx.AsyncClient stays alive between calls, so HTTP connections remain pooled.