Provide a convenience method for using the default Checkstyle configuration with Gradle

[wilkinsona]

Maven's Checkstyle plugin supports loading configuration from the classpath so Java Format's can be used by setting configLocation to point to the io/spring/javaformat/checkstyle/checkstyle.xml classpath resource. Gradle's Checkstyle plugin has no equivalent feature. At the moment, this means that you have to provide your own checkstyle.xml, even if it's identical to the default config file.

We could improve this situation by provided a convenience method that configures Checkstyle with Java Format's default config:

springJavaFormat {
    checkstyle {
        applyDefaultConfig()
    }
}