Reduce Iceberg snapshot watcher thread use

[1fanwang]

Each stateful Iceberg snapshot watcher submits worker-thread jobs to load and save its watermark. A triggerer running many watchers spends executor capacity on storage that already has an async API.

After this change, stateful watchers await watermark storage directly. Worker threads remain limited to synchronous PyIceberg catalog calls. Watchers without an injected state store keep their existing behavior.

Depends on #72127 and must merge after it.

Test output

$ uv run --project providers/apache/iceberg pytest providers/apache/iceberg/tests/unit/apache/iceberg/triggers/test_iceberg.py -xvs
# BeforeFAILED providers/apache/iceberg/tests/unit/apache/iceberg/triggers/test_iceberg.py::test_resumes_from_the_stored_watermarkE AssertionError: assert [...] == []1 failed, 10 passed
# After19 passed, 1 warning in 17.57s

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