support for falling back on other decoders if unsuccessfull

[berndpfrommer]

This PR:

• adopts the new ffmpeg_encoder_api from this PR which allows for probing of decoders. The decoder parameter can now contain a list of comma-separated decoders which will be tried in order.
• adds tests to the repo
• reformats to the black python formatter
• adds arguments to some example launch files
• allows arbitrary AVOptions setting via av_option
• removes parameters "preset", "tune", "delay", and "crf" (must now be set via "av_options")
• removes some default values (like bit_rate) to force user to set them explicitly
• changes the parameter names: remove the leading ".", so now the parameters are named specify "camera.image_raw.." as opposed to ".camera.image_raw..."

[hidmic]

First pass, neat. Testing locally.