Distribute as Flatpak at Flathub

**Pau Rodriguez-Estivill** ([Bug 66258](https://bz.apache.org/bugzilla//show_bug.cgi?id=66258&redirect=false)):
Flathub hosts user applications using the Flatpak distribution method.

Flatpak allows applications to be compatible with almost all linux distros because they are compiled and run in a sandbox environment.
You can know more about flatpak and flathub at https://flatpak.org/ and https://flathub.org/

As an example/template I did some tests here but I don't submit the PR back to Flathub for now as Flathub prefers the main author to perform the actual publication and maintenance of the applications.

https://github.com/prodrigestivill/flatpak/tree/org.apache.jmeter

OS: Linux