Add an openh264 extension
Adds an openh264 extension, but does not publish it to flathub. This is so we can check that things don't churn too much causing lots of downloads of the same thing. See !1638 (merged).
I have confirmed that older flatpaks fail gracefully when they try to install this extension, so we can leave it as autodownload.
Part of #854 (closed)
Edited by Javier Jardón