Add `FFmpeg` dynamic application build

Add the FFmpeg applications as a dynamic build. Test it using app-elfloader. If any problems appear, open issues on the Unikraft core repository.

The typical process of adding apps to the dynamic-apps repository is:

Create the app directory that will store: the executable, depending shared libraries, configuration files, data files, etc.
Use apt download to download the package(s) (.deb)
Extract the executable, configuration and data files required to run the application.
Update the configuration file, if required.
Run the executable locally, to test it.
Run the extract.sh script to collect the necessary shared libraries:
Now profit. Use app-elfloader.

Hackathon points: 20

unikraft / dynamic-apps