oggopus: provide bitrate

[lazka]

We should try to guess a bitrate that isn't depended on the metadata size.

Might also be good to add a generic test which tests that the bitrate doesn't change with all metadata removed.

[Goldmaster]

hi there, is there any update on this issue with opus files? https://community.metabrainz.org/t/is-lossless-plugin-no-longer-working-with-m4a/469429/6