Move the build to gradle?

Created by: jmini

As discussed in #162, our builds gets more and more complex and takes to long on small hardware.

If we switch to gradle:

• Build time will be improved (gradle is built to be more performant and to build only changes)
• We can remove the maven-calls-graddle workaround used to build openapi-generator-gradle-plugin
• We benefit for all improvements made in the build tooling area (maven does not evolve that much)
• Optionally, if we find a server to host "Build cache" similar to what Gradle Enterprise provides, we can also leverage this technology. Maybe bintray can be used, it is free for Open Source. I found an article about using Artifactory as distributed build cache. Bintray is a sort of managed Artifactory from the same software editor. Features are similar.