Fail Kotlin sample compilation if API classes stop to be public

Serves as a 'regression test'.

Author: pkubowicz

samples/client/petstore/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/usage/AspectJLikeUse.kt

@@ -0,0 +1,7 @@
+package org.openapitools.usage
+
+import org.openapitools.client.apis.PetApi
+import org.springframework.web.client.RestClient
+
+// 'regression test' that API classes are open https://github.com/OpenAPITools/openapi-generator/issues/22271
+class AspectJLikeUse(client: RestClient) : PetApi(client)