Alumna::FieldDescriptor

new(name : String, type : FieldType, required : Bool = true, nullable : Bool = false, unique : Bool = false, indexed : Bool = false, read_only : Bool = false, min_length : Int32 | Nil = nil, max_length : Int32 | Nil = nil, format_name : String | Nil = nil, format_validator : Proc(String, Bool) | Nil = nil, format_message : String | Nil = nil, required_on : Array(ServiceMethod) | Nil = nil, sub_schema : Schema | Nil = nil, element_type : FieldType | Nil = nil, default : AnyData | Proc(AnyData) | Unprovided | Nil = Unprovided.new)