Add Zstandard compression to avro file format

I'd like to add Zstandard compression for Avro. 

At compression level 1 It is almost as fast as Snappy at compression, with compression ratios more like gzip.  At higher levels of compression, it is more compact than gzip -9 with much lower CPU when compressing and roughly 3x faster decompression.

Adding it to Java is fairly easy.  We'll need to say something about it in the spec however, as an 'optinal' codec.