Support Gradle Toolchains

[DJViking]

Support using a custom toolchain when running the MavenExec task

task('createWebstart', type: MavenExec) {
    javaLauncher = javaToolchains.launcherFor {
        languageVersion = JavaLanguageVersion.of(8)
    }
    goals 'clean', 'install'
}

As documentet here https://docs.gradle.org/current/userguide/toolchains.html#specify_custom_toolchains_for_individual_tasks

Could not set unknown property 'javaLauncher' for task ':createWebstart' of type com.github.dkorotych.gradle.maven.exec.MavenExec.

Toolchains for plugin authors

[DJViking]

Thare are some MavenOptions for setting Toolchains

https://github.com/dkorotych/gradle-maven-exec-plugin/blob/master/src/main/java/com/github/dkorotych/gradle/maven/MavenOptions.java#L588

https://github.com/dkorotych/gradle-maven-exec-plugin/blob/master/src/main/java/com/github/dkorotych/gradle/maven/MavenOptions.java#L1080

However no documentation how this works.