Added support for patterns on model properties (#2948)

Author: ingvirafn

modules/openapi-generator/src/main/resources/aspnetcore/2.1/model.mustache

@@ -26,6 +26,9 @@ namespace {{modelPackage}}
         {{#required}}
         [Required]
         {{/required}}
+        {{#pattern}}
+        [RegularExpression("{{{pattern}}}")]
+        {{/pattern}}
         [DataMember(Name="{{baseName}}", EmitDefaultValue={{#isNullable}}true{{/isNullable}}{{^isNullable}}false{{/isNullable}})]
         {{#isEnum}}
         public {{{datatypeWithEnum}}}{{#isEnum}}{{^isContainer}}?{{/isContainer}}{{/isEnum}} {{name}} { get; set; }