WSSink: evolve the automatic schema as new data pours in#70
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This would simplify wsds/WSSink usage for one-off datasets (evals, synthetic data).
Currently we depend only on the first batch of data to determine the schema. When we detect new columns we silently ignore them(sic!) and when there are datatype changes we error out.
If we come up with a set of rules for automatic casting (PyArrow defaults do work but are not great in many cases) and other edge cases, we can rewrite the data written so far using a new schema and avoid most conflicts.
One especially nice improvement is automatic support for large binary without an explicit declaration.