Provide JDK 1.8 (backward) compatibility of helix-core

[himanshukandwal]

Is your feature request related to a problem? Please describe. We have some consumers which are still running their (open-source) systems on JRE-8, hence they are not able to run helix libraries compiled with JDK11+.

Describe the solution you'd like We would like to provide a backward compatible support to our consumers where they also have an option to use JDK-8 compiled helix-core jar, if they have such a requirement. By default we will generate JDK-11 jars and JDK-8 jars using a classifier.

<major.minor.patch> will use JDK11 (default)
<major.minor.patch> classifier say jdk8 , will use JDK8

For JDK-11 usecase

      <dependency>
          <groupId>org.apache</groupId>
          <artifactId>helix</artifactId>
          <version>1.3.4</version>
      </dependency>

For JDK-8 usecase

      <dependency>
          <groupId>org.apache</groupId>
          <artifactId>helix</artifactId>
          <version>1.3.4</version>
          <classifier>jdk8</classifier>
      </dependency>

Additional context Add any other context or screenshots about the feature request here.