sout: add a tracer utility stream filter

The VLC tracing API is really useful in a lot of debugging scenario. It was previously implemented in playback but lack an implementation in the stream output chain.

This patch propose a named stream filter that can be placed before any other module and trace their input. It allow the user to choose precisely at which node of the chain they want tracing and under which name.

An example usage can be found in the help section of the module:

#autodel:trace{name=transcode}:transcode{}:trace{name=file}:file{}

This output a standard trace than can be read by a plotting software (vlc-pa in this case):