Add zstd as a compression option?

would there be interest in adding zstd to partd. At lowish compression levels I've found it to have better compression and around twice as fast as snappy.

The issue names no files or tests. Start by tracing partd's existing compression options and Snappy integration, then identify how dependencies and selectable options are defined. Done means zstd can be selected and works alongside the existing compression choices.

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