Support multi-module Maven projects in the Liberty dashboard.

[mrglavas]

The overall goal of this epic is to add support for multi-module Maven projects to Liberty Tools for VSCode, enhancing the Liberty dashboard to provide an aggregator-centered view of the modules and extending the full set of Liberty actions to function with multi-module Maven projects.

LTV will be responsible for modelling the multi-module relationships and keeping them updated.

As a first milestone, we should develop a working demo for the following scenario and get feedback from the wider team before proceeding further:

• Load the Liberty guides multi-module application https://openliberty.io/guides/maven-multimodules.html into the Liberty dashboard. (Only the aggregator project should be displayed in the view.)
• Execute the Start action for the Liberty EAR module from the aggregator project/module with the appropriate command line (e.g. mvn liberty:dev -pl :guide-maven-multimodules-ear -am).

See the Liberty Tools for Eclipse epic for reference: OpenLiberty/liberty-tools-eclipse#398. The mock screenshots below are for Eclipse but should help in visualizing the UI for Liberty Tools for VSCode.