Add @serializable annotation to enum for kotlinx-serialization

Author: yzaoui

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/api.mustache

@@ -77,6 +77,7 @@ import okhttp3.MultipartBody
     /**
     * enum for parameter {{paramName}}
     */
+    {{#kotlinx_serialization}}@Serializable{{/kotlinx_serialization}}
     {{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}enum class {{enumName}}{{operationIdCamelCase}}({{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val value: {{^isContainer}}{{dataType}}{{/isContainer}}{{#isContainer}}kotlin.String{{/isContainer}}) {
     {{^enumUnknownDefaultCase}}
         {{#allowableValues}}