Fix missing @Serializable on enums for kotlin/jvm-retrofit2 (OpenAPITools#21907)

* Add @serializable annotation to enum for kotlinx-serialization

* Adjust template, generate samples

* Fix template

Author: yzaoui

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

@@ -77,7 +77,8 @@ import okhttp3.MultipartBody
     /**
     * enum for parameter {{paramName}}
     */
-    {{#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}}) {
+    {{#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}}
             {{#enumVars}}

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -44,6 +44,7 @@ interface PetApi {
     /**
     * enum for parameter status
     */
+    @Serializable
     enum class StatusFindPetsByStatus(val value: kotlin.String) {
         @SerialName(value = "available") AVAILABLE("available"),
         @SerialName(value = "pending") PENDING("pending"),