uber-jar the frameless sample and set mainClass

[prabhah]

What kind of change does this PR introduce? (check at least one)

• [] Bugfix
• Feature
• Code style update
• Refactor
• [x ] Build-related changes
• Other, please describe:

The description of the PR:

1. missing "mainClass" definition in the pom.xml

java -jar failed:

java -jar ./xxl-job-executor-samples/xxl-job-executor-sample-frameless/target/xxl-job-executor-sample-frameless-2.4.1-SNAPSHOT.jar
no main manifest attribute, in ./xxl-job-executor-samples/xxl-job-executor-sample-frameless/target/xxl-job-executor-sample-frameless-2.4.1-SNAPSHOT.jar

this method works,

java -cp ./xxl-job-executor-samples/xxl-job-executor-sample-frameless/target/xxl-job-executor-sample-frameless-2.4.1-SNAPSHOT.jar com.xxl.job.executor.sample.frameless.FramelessApplication

2. didn't pack the dependence's jars to the final jar, difficult to run the sample

java -cp ./xxl-job-executor-samples/xxl-job-executor-sample-frameless/target/xxl-job-executor-sample-frameless-2.4.1-SNAPSHOT.jar com.xxl.job.executor.sample.frameless.FramelessApplication
Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
	at com.xxl.job.executor.sample.frameless.FramelessApplication.<clinit>(FramelessApplication.java:13)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
	at java.net.URLClassLoader.findClass(URLClassLoader.java:387)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:351)

Other information: