[Dart] Add configuration to add new generators and add json_serializable (#8789)

* Add Dart configuration flag to set new generator

* Change templates to add json_serializable generator

* Fix dart tests

* Add custom serialization for standard generator

* this change opens us a way to allow adding more than 2 generators

* Add sample project for json_serializer

* Use basename for json field name

* Update samples with basename as json key

* Update dart generator docs

* Create AbstractDartCodegen class

* Update docs, dart-dio and dart-jaguar dont get json_serializable yet

* Deduplicate method calls

* Dix dart-dio generation

* Revert formatting change in dart mustache

* Fix formatting changes again

* Improve dart serialization documentation

specify default serializer

* Better name for cli option

* Rename default dart generator to 'native'

* Another attempt to reduce whitespace noise

* Fix api-client and api-helper

* Rename default serialization mode to include word serialization

* Remove trailing whitespace in enums

docs/generators/dart.md

@@ -19,6 +19,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |pubLibrary|Library name in generated code| |null|
 |pubName|Name in generated pubspec| |null|
 |pubVersion|Version in generated pubspec| |null|
+|serializationLibrary|Specify serialization library|<dl><dt>**native_serialization**</dt><dd>Use native serializer, backwards compatible</dd><dt>**json_serializable**</dt><dd>Use json_serializable</dd></dl>|native_serialization|
 |sortModelPropertiesByRequiredFlag|Sort model properties to place required parameters before optional parameters.| |true|
 |sortParamsByRequiredFlag|Sort method arguments to place required parameters before optional parameters.| |true|
 |sourceFolder|Source folder for generated code| |null|