[enhancement] Default middleware for handling validation exceptions

[mechite]

avaje-http provides a validation API, where avaje-validator among others are available.
Having avaje-http automatically register, for example, the following, could add convenience:

jex.error(ValidationException.class, (context, exception) -> {
      List<ValidationException.Violation> violations = exception.getErrors();

      int violationCount = violations.size();
      String violationList = violations.stream()
          .map(violation -> "'" + violation.getField() + "' " + violation.getMessage())
          .collect(joining("\n"));

      // return a plain-text error message
      context.write(format("Bad Request [{0} validation violations: \n{1}]", violationCount, violationList));
});

1. avaje-http provides an agnostic io.avaje.http.api.ValidationException among each validator implementation.

   Therefore, this can be used by the generator, and assumed to exist in all cases.
   If a user does not have a validator, the error-handling middleware is just a hidden no-op they won't notice.

2. The above example displays a plain-text error message.

3. When avaje-jsonb is available (or just avaje-json-core really):
   RFC 7807 / RFC 9457 compliant message as follows can be emitted from the ValidationException:

   {
    "type": "https://avaje.io/validator/validation-error-message",
    "title": "Your request is not valid.",
    "errors": [
                {
                  "detail": "must be a positive integer",
                  "pointer": "#/age"
                },
                {
                  "detail": "must be 'green', 'red' or 'blue'",
                  "pointer": "#/profile/color"
                }
             ]
   }

   (this is a really bad, rough example)

4. Javalin offers a default middleware for their Validator as well.

   app.get("/test123/{value}", ctx -> {
       int value = ctx.queryParamAsClass("value", Integer.class)
           .check(q -> (q > 0), "Quantity must be positive")
           .get();
       ctx.result(value);
   });

   Below is what their middleware looks like:

   app.exception(ValidationException::class, (error, ctx) ->
       ctx.json(e.errors).status(400);
   });

[SentryMan]

I vibe with this