AsyncValidator stream

Currently `AsyncValidator::validate_value()` evaluates all validations using `join_all` and waits for them all to complete. It would be nice to also have an option to evaluate them all as a stream (perhaps `validate_value` could use this under the hood for code reuse, and collect them all), so that validations can be used as they arrive. Errors can be displayed before all validations have completed, so the user can react quickly. Then it would also be possible to only display the error from the first validator. It opens up a lot of options.

Start with AsyncValidator::validate_value() and its current join_all-based evaluation. Review the surrounding validation API to determine how streamed results should be exposed and how existing collection behavior can remain available. Done means validations can be consumed as they complete, with early error handling supported and existing validation behavior preserved.

Written by the indexing model from the issue text.