compress/snappy: configurable compression level

[chriso]

There are three snappy compression routines provided by https://github.com/klauspost/compress. See https://github.com/klauspost/compress/tree/master/s2#readme for more information.

At the moment kafka-go is hard-coded to use snappy.Encode aka. s2.EncodeSnappyBetter.

This PR makes the compression level configurable, allowing you to choose either a faster compression routine (s2.EncodeSnappy) or a slower one that compresses better (s2.EncodeSnappyBest).

There appears to be a global set of codecs shared by all producers. To set the compression level for all producers (configured to use snappy) you'd need to do something like this:

compress.SnappyCodec.Compression = snappy.FasterCompression

This is the same approach taken by the gzip codec and zstd codec, although in those cases they accept an integer level. Ideally we'd be able to set the compression level per producer, but it looks like a much larger refactor would be required.

[achille-roussel]

Nice addition, thanks for the contribution!