Add extension directory for external tools

Using the flatpak for a few weeks now and the only drawback I see from something like the visual studio code extension is the lack for a way to easily package additional tools. The visual studio code flatpak adds this support with:

1. a directory extension making it possible for additional software to be run in the flatpak environment (useful to run containers with podman-remote for example)
2. a host-spawn command to easily run a shell on the host

I think the extension directory would be an easy way to add more usage of the package without having a great cost in support.