Add support for typealiases in Kotlin client (#8084)

Author: PatrickNLT

modules/swagger-codegen/src/main/resources/kotlin-client/model.mustache

@@ -9,6 +9,6 @@ import org.threeten.bp.LocalDateTime
 
 {{#models}}
 {{#model}}
-{{#isEnum}}{{>enum_class}}{{/isEnum}}{{^isEnum}}{{>data_class}}{{/isEnum}}
+{{#isEnum}}{{>enum_class}}{{/isEnum}}{{^isEnum}}{{#isAlias}}typealias {{classname}} = {{dataType}}{{/isAlias}}{{^isAlias}}{{>data_class}}{{/isAlias}}{{/isEnum}}
 {{/model}}
 {{/models}}