Add a new variable `CPACK_DEBIAN_COMPRESSION_LEVEL` to control the compression
level used when generating Debian packages. This complements the existing
`CPACK_DEBIAN_COMPRESSION_TYPE` variable and allows finer control over the
compression performance vs. size trade-off. Supported values correspond to
the compression level accepted by the underlying compressor (e.g. gzip, xz,
zstd, etc.).