Provide a way to specify a value transformation function applies to the data passed to onSubmit

When asking https://stackoverflow.com/q/70774121/298479 I found another question where a comment (even from @erikras IIRC) suggested storing the "raw" data in the value and converting it only during submission. However, there is no clean way of doing that without keeping fields self-contained. For example, with a date field any "custom" format (e.g. locale-specific) should be handled within the field (the value would always be iso format), and for a number field I may need to deal with strings at first to provide decent validation, but the final data should be a number.

My suggestion is to add something similar to `format` but for the data to be submitted, e.g. `transformValue` or `formatForSubmit` (names aren't amazing but not relevant for now).

That way `format` and `validate` would still have access to the raw data, but form-level validators and onSubmit would receive data in the format the field wants to "output".