TestJars support

[BoykoAlex]

The idea is to make the app having TestJars on the classpath launched via the IDE aware of GAVs, classpath and main classes of projects residing in the same workspace. Thus TestJar support won't attempt to load these projects from maven but instead use workspace versions of these.
See spring-projects-experimental/spring-boot-testjars#15 for more details

[BoykoAlex]

The plan is the following:

1. Launch creation is initiated on the IDE client side.
2. IDE client executes a command on the LS side to receive a list of "Executable" Boot Projects where ExecutableBootProject looks like:

interface ExecutableBootProject {
  name: string;
  uri: string;
  bootAppClass: string;
  classpath: string[]
  gav: string; // <groupId>:<artifactId>:<version>
}

The data is available on the LS in memory hence should be rather fast to arrive
3. Client saves this data in property files somewhere (temp folder? workspace folder?)
4. Updates launch with TestJar needed environment variables. (Would be nice if we could avoid saving into files... seems somewhat redundant. Perhaps it can be packed into 2 environment variables...)

[BoykoAlex]

TestJars support is for maven projects only so far: 52dd4d2

[BoykoAlex]

Preference setting went in with d3af0a3.
Gradle projects GAV and hence the TestJars support to be delivered with #1201 in 4.22.1. The rest is done for 4.22.0