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).

[yara-blue]

Since this only works for symphonia decoders this should maybe be named with_symphonia_decoder. On the other hand I see a near future where we drop all the other decoders (I have not seen any issue with the Symphonia decoders and they have been the default for a while). If we do that the name is fine.

[aschey]

I agree, that makes sense. Updated 👍

[yara-blue]

Looks great to me! There is a trivial conflict, I think your explanation about features deserves to live in the toml, and we might wanna think about the method name for a sec. As soon as that's done LGTM.

PS: apologies for the somewhat slow review. Thanks for picking this up and keeping the ball rolling.

PPS: it's soo great to see these old issues closed :)

[yara-blue]

Yay 🎉