Support `minLength`/`maxLength` for non-required strings

[dta5003]

Currently if a parameter is defined like { type: string, minLength: 2, maxLength: 2 }, but the parameter is not required, it results in a runtime error. The System.ComponentModel.DataAnnotations.MinLengthAttribute.IsValid(Object value) throws since it is attempting to test the IOptional<string> rather than the string.

[mdekrey]

Oh, interesting; this may require adding new validation attributes, custom validators, etc. for any IOptional<> values.