Add SoCo method to determine HDMI/Optical input audio format

[rytilahti]

This is extracted from #780 to avoid overloading that PR with multiple different features.

This PR adds two new properties to the device to report the audio input format (e.g., if there is no input and whether the input is dolby digital 5.1 or just PCM stereo) for toslink/hdmi input found on soundbars:

• soundbar_audio_input_format_code to return the audio input format as reported by the device, and
• soundbar_audio_input_format to return a human-readable description of the format, if available.

I'm not completely happy about this:

• The mapping is very incomplete, but contains log output to simplify extending it in the future.
• Some values found in the internet (https://en.community.sonos.com/advanced-setups-229000/getzoneinfo-htaudioin-codes-6750870) do not match to the ones I'm seeing on my system.
• Maybe this should be an enum instead of a string?

[coveralls]

Coverage decreased (-1.6%) to 52.613% when pulling 023a8d7 on rytilahti:feat/soundbar_audio_input_format into bb71285 on SoCo:master.

[rytilahti]

Added some tests & did some fine tuning. @pwt please let me know if there's some other changes that are still needed to get this merged!