feat: add support for compression level -2 (HuffmanOnly)

[kruskall]

The docappender is rejecting -2 as a value for CompressionLevel despite it being a valid value for gzip compression in the standard library.

The validation logic should be updated to accept -2:

go-docappender/appender.go

Line 86 in c563b18

if cfg.CompressionLevel < -1 || cfg.CompressionLevel > 9 {