Implement skeleton of native litr-muxers module

[IanDBird]

Part of https://github.com/linkedin/LiTr/pull/235

This first PR does the following:

• New module called litr-muxers to contain the native muxers module (based upon libavformat)
• Build instructions and scripts to locally build the required libavformat (and dependencies)
• Fallback support to binaries that are contained in the repo, if the consumer prefers not to build them themselves
• Some basic headers that will be required as part of the native code.