Allow user to choose the address of the MicroProfile REST service

[moghaddam]

The IntelliJ plugin for Spring Initializr and also the open source Quarkus plugin that released recently, both allow developers in the very first step to choose the REST API endpoint that they wants to use to generate the code. This allows developers who don't have direct access to the Internet from their computer to use a proxied address to access the API and use the plugin. Also this would enable them to use a custom version of the API (which can also be useful during the process of development and test of the plugin with new API versions without updating the code).