feat: opus support via libopus and allow registering additional codecs

[aschey]

Supersedes #806

Resolves #38

Thanks @royalmustard for starting on this! I took the changes from the previous PR and made a few tweaks:

• Exposes a method to add a decoder to the existing registry instead. This is more difficult to misuse because it's easy to forget to re-enable the default codecs when creating a completely new registry.
• Adds opus support via the symphonia-libopus flag. Opus support is a common enough request that I think it makes sense to support this via a flag rather than require users to add the codec manually.
• Bumps the MSRV so CI can pass (discussed here).
• Adds a different third-party codec example using symphonia-adapter-fdk-aac (FDK supports HE-AAC which is currently not supported by Symphonia's AAC implementation).

[aschey]

Note that there are a lot of places that use #[cfg(feature = "symphonia")] even though there was no feature named symphonia. A feature with this name was created implicitly due to the other features that reference symphonia/xxx. For symphonia-libopus, I wanted to make sure the base symphonia feature was also enabled, so we need to make this feature explicit now.