build.gradle: add spotless plugin

This adds the build support for formatting-checks
in CI. Run with:

./gradlew spotlessCheck -PclangFormat=clang-format-15

to use a different clang-format binary, or put:

clangFormat = clang-format-15

in gradle.properties.

.gitignore

@@ -2,5 +2,6 @@
 .gradle_home
 *~
 build
+gradle.properties
 /dist
 /javadoc

build.gradle

@@ -1,5 +1,6 @@
 plugins {
-    id 'application'
+  id 'application'
+  id 'com.diffplug.spotless' version '6.22.0'
 }
 
 group = 'org.contikios.cooja'
@@ -12,6 +13,31 @@ java {
   }
 }
 
+spotless {
+  java {
+    toggleOffOn()
+    // Removing unused imports fails with unterminated string literal in
+    // GUI.java. The line is a switch statement with a string case, so
+    // possibly something to do with Java 17.
+    // removeUnusedImports()
+    importOrder()
+    // trimTrailingWhitespace()
+    // cleanthat()
+    // formatAnnotations()
+
+    // Use a different clang-format binary with -PclangFormat=clang-format-15.
+    def clangFormatBinary = (String) project.findProperty('clangFormat') ?: 'clang-format'
+    def clangOutput = new ByteArrayOutputStream()
+    exec {
+      commandLine clangFormatBinary, '--version'
+      standardOutput = clangOutput
+    }
+    def clangVersion = (String) (clangOutput.toString() =~ /\d+\.\d+\.\d+/)[0]
+    // FIXME: enable clangFormat in the future.
+//    clangFormat(clangVersion).pathToExe(clangFormatBinary).style('file')
+  }
+}
+
 repositories {
   mavenCentral()
 }