Refactor: Separate RDCU configuration into its own struct

Previously, both hardware and (chunk) software compression shared the same
struct for configuration. To simplify usage, RDCU compression now has
its own separate configuration struct (`rdcu_cfg`), while the software
(chunk) compression maintains its own private configuration. The
function `compress_like_rdcu()` was added to simulate the functionality
of the RDCU hardware compressor.