Descripe the feature and motivation

The environment variables OPENCV_FFMPEG_CAPTURE_OPTIONS and OPENCV_FFMPEG_WRITER_OPTIONS potentially have great impact on the internals of the ffmpeg video backend. but when they are used the log files don't show it.
I'd like to print a debug message containing the value of those variables (in case they are set).

Additional context

This issue #22622 contains examples of how those variables can be used.