[kotlin-client] add spotless plugin (#16645)

* add spotless plugin to kotlin client

* add spotless plugin to kotlin client

* use 6.13.0

samples/client/others/kotlin-jvm-okhttp-parameter-tests/build.gradle

@@ -8,22 +8,46 @@ wrapper {
 
 buildscript {
     ext.kotlin_version = '1.8.10'
+    ext.spotless_version = "6.13.0"
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }
     }
     dependencies {
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+        classpath "com.diffplug.spotless:spotless-plugin-gradle:$spotless_version"
     }
 }
 
 apply plugin: 'kotlin'
 apply plugin: 'maven-publish'
+apply plugin: 'com.diffplug.spotless'
 
 repositories {
     maven { url "https://repo1.maven.org/maven2" }
 }
 
+// Use spotless plugin to automatically format code, remove unused import, etc
+// To apply changes directly to the file, run `gradlew spotlessApply`
+// Ref: https://github.com/diffplug/spotless/tree/main/plugin-gradle
+spotless {
+    // comment out below to run spotless as part of the `check` task
+    enforceCheck false
+
+    format 'misc', {
+        // define the files (e.g. '*.gradle', '*.md') to apply `misc` to
+        target '.gitignore'
+
+        // define the steps to apply to those files
+        trimTrailingWhitespace()
+        indentWithSpaces() // Takes an integer argument if you don't like 4
+        endWithNewline()
+    }
+    kotlin {
+        ktfmt()
+    }
+}
+
 test {
     useJUnitPlatform()
 }