Add mutable model option to kotlin generators (#4115)

* Add mutable model option to kotlin generators

fix #3803

* doc

* Change template name to modelMutable

* Samples

samples/server/petstore/kotlin-springboot-modelMutable/settings.gradle

@@ -0,0 +1,15 @@
+pluginManagement {
+	repositories {
+		maven { url = uri("https://repo.spring.io/snapshot") }
+		maven { url = uri("https://repo.spring.io/milestone") }
+		gradlePluginPortal()
+	}
+	resolutionStrategy {
+		eachPlugin {
+			if (requested.id.id == "org.springframework.boot") {
+				useModule("org.springframework.boot:spring-boot-gradle-plugin:${requested.version}")
+			}
+		}
+	}
+}
+rootProject.name = "openapi-spring"