[Rpm-maint] [rpm-software-management/rpm] API improvement to accommodate for RPM CoW (PR#1470) (Discussion #2057)

[David Disseldorp]

I hacked together a proof of concept implementation which uses `BTRFS_IOC_ENCODED_WRITE` to write a zstd-compressed cpio payload directly to disk as-is, from a carefully aligned rpm. Compressed extents are then reflinked to the installation path.
presentation: https://www.youtube.com/live/qNGw8IKaqc0?si=vrLkk8Bi9Odfqm4Z&t=10325
rpm-rs based source: https://github.com/ddiss/rpm/tree/poc-btrfs-zstd-encoded-io-reflink-extract

The rpm payload format isn't modified, although there's a slight "bending" of the cpio/newc spec to use the filename field for padding. zstandard frames making up the compressed rpm payload explicitly carry both compressed and uncompressed lengths, to allow detection of filesystem-supported I/O sizes.

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