Add support for Zstd coders

[kellen]

Adds

• saveAsZstdDictionary to train a Zstd dictionary on some arbitrary SCollection[T]. Estimates the average size of elements T, collects n elements based on a target training set size, then trains and saves the Zstd dictionary.
• A scala ZstdCoder object with transform Coders for the simple T or for each side of a (K, V)
• command line argument to map from a type to a dictionary, causing instances of MyClass to get Zstd compression automagically. Probably fails if the type is parameterized. --zstdDictionary=com.spotify.scio.MyClass:gs://bucket/path/dict.bin