Add basic support for reading average bitrates

[codedcosmos]

Bitrate is seemingly missing from all song metadata in this project. Even songs that contain the bitrate in the header (alac, mp3, possibly more?) don't make this information available to the user.

This commit stores the bitrate in CodecParameters as "average_bitrate". This is because some formats like flac have a variable bitrate. It's better to provide something than to promise a constant bitrate and be technically wrong as some decoders do.

This average bitrate is currently only implemented for mp3. But possibly other codecs might want to implement it. This change is fully backwards compatible.

[codedcosmos]

Hi 👋
This is honestly my first pull request, so sorry if I have missed something or done something wrong. I'll try to help you resolve anything that I've missed 🙂 , But I did make sure to read the contributing guidelines page first.

I'm not sure if this repo has automated testing, but I'll write here that I I did run cargo test and cargo fmt --all before pushing.

[sscobici]

You might want to consider rebasing this PR onto the dev-0.6 branch.
See details here: #330

[codedcosmos]

I'll take a look, do you mind if I also implement bitrate for other protocols that store that data in the header?

WAV for example stores it as byterate (and can easily enough be divded by 8 to get bitrate).