Refactor the `as_validation_exception_field` function definition in...

Refactor the `as_validation_exception_field` function definition in `ServerBuilderConstraintViolations`. (#3719)

This PR refactors the code to limit the visibility of
`crate::model::ValidationExceptionField` to only those decorators and
associated classes that are invoked when
`smithy.framework#ValidationException` is being used.

More information is in [this
issue](https://github.com/smithy-lang/smithy-rs/issues/3718).

Closes: [3178](https://github.com/smithy-lang/smithy-rs/issues/3718

)

---------

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>
Co-authored-by: david-perez <d@vidp.dev>