Implement `codecs` attribute for `podcast:alternateEnclosure`

This could simply be deserialized to a string (or a vector of strings) but a vector of enums would work better. For that, [`codecs` syntax](https://www.rfc-editor.org/rfc/rfc6381#section-3.2) would have to be carefully interpreted.

Could [this](https://github.com/dholroyd/rfc6381-codec) be utilised?