add FindFFMPEG

Since I started a big OpenCV rework for vcpkg (the specific port was almost non-working for my configuration), I found myself writing or improving many modules for CMake related to many dependencies. In order also to ease the porting mainteniance effort, having those modules accepted upstream I think can be even more beneficial.

In this MR, there is the first module, which is a new entry, related to ffmpeg libraries.

Please let me know if it is interesting, if any change is necessary, and so on.