add generator template support for dart2-api

Author: robertcsakany

openapi-generator-annotations/lib/src/openapi_generator_annotations_base.dart

@@ -18,7 +18,7 @@ class Openapi {
   /// -t
   final String templateDirectory;
 
-  /// Generator to use (dart|dart-jaguar|dart-dio)
+  /// Generator to use (dart|dart2-api|dart-jaguar|dart-dio)
   ///
   /// -g, --generator-name
   final String generatorName;

openapi-generator/lib/src/openapi_generator_runner.dart

@@ -49,9 +49,10 @@ class OpenapiGenerator extends GeneratorForAnnotation<Openapi> {
         _readFieldValueAsString(annotation, 'generatorName', 'dart');
     if (generator != 'dart' &&
         generator != 'dart-dio' &&
+        generator != 'dart2-api' &&
         generator != 'dart-jaguar') {
       throw InvalidGenerationSourceError(
-        'Generator name must be any of dart, dart-dio, dart-jaguar.',
+        'Generator name must be any of dart, dart2-api, dart-dio, dart-jaguar.',
       );
     }
     command = '$command$separator-g$separator$generator';