Support specifying explicit/separate JRE/JDK for a launch task

[tginiotis-at-work]

Currently the JDK/JRE that VSCode uses to launch a java type task from launch.json seems to be picked via some internal VSCode logic. This prevents from using different JDK/JREs for compile-time and run-time.

It would be useful to allow to explicitly define a java home in the launch configuration to be used for run-time.

Use-case: I am working on code in an aarch64 environment. The code can be compiled just fine with an aarch64 JDK, but for runtime, I want to launch it with an x86_64 emulated JDK, so it could load some dependent native x86_64 libs in run-time and etc.

I am able to compile with one JDK and launch the code with the emulated other arch JDK with some configuration juggling, but as you can imagine - that is far from ideal. A key in a the launch configuration specifying the JDK/JRE would be ideal. Other IDEs, like Eclipse and IntelliJ, allow specifying a separate run-time JDK/JRE too.

Might be related: #884

[testforstephen]

In launch.json, we already support a settting "javaExec" to allow you specify a different java.exe to run the application.

[tginiotis-at-work]

Indeed, thanks! Could not find it documented anywhere.

[testforstephen]

Added to the README doc.