Initialize converters from type annotations

We could initialize converters from type annotations instead of the spec in the template.
This would be an optional feature that would probably need to be disabled by default in order to avoid breaking change.
The annotations would need to be consistent across all responders for the same parameter name, otherwise an instance of `ValueError` or subclass would be raised.

No files, tests, or entry points are identified. First clarify the design for deriving converters from annotations, including the default-disabled behavior, consistency checks across responders, and the expected error cases; completion should include agreed behavior and coverage for these cases.

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