Reconsider to extend the JvmBuildTarget

[jdneo]

Currently the definition of JvmBuildTarget is:

export interface JvmBuildTarget {
  /** Uri representing absolute path to jdk
   * For example: file:///usr/lib/jvm/java-8-openjdk-amd64 */
  javaHome?: URI;

  /** The java version this target is supposed to use.
   * For example: 1.8 */
  javaVersion?: string;
}

We can use javaHome to be the JDK path used to launch the project, and javaVersion to be the version of javaHome.

This implies that it is the server's responsibility to search the proper JDK to launch the Gradle daemon.

Based on that, add following fields:

• sourceCompatibility - source level
• targetCompatibility - target level

  Note that, sourceCompatibility and targetCompatibility might be different values so we can not use javaVersion to represent them both.

• compileArgs - arguments used for compilation.

@testforstephen WDYT?