Add external codecs to extras_require

The Python implementations support some compression algorithms that aren't supported in the python standard library. Users would need to install these codecs themselves to use them. To make that process easier, we can add these optional codecs to extras_require in the setup scripts so that users can require `avro[zstandard]` instead of separately installing `avro` and `zstandard`.

There's precedent here, since this is how avro already does it for snappy. Also, encouraging installing zstandard or snappy via extras_require can reduce the risk of compatibility issues.