export-tar: support sparse files

[enkore]

https://www.gnu.org/software/tar/manual/html_node/PAX-1.html

This is only possible in a fairly limited fashion, because the sparse block mapping is prepended to the file contents. Therefore, runs of zeroes cannot be folded while writing the file contents. Instead, zero chunks (of the maximum length allowed by the archive.metadata[chunker_params], as generated by the chunker) can be picked from the chunk ID list before processing the main file contents. Then, the appropriate tar block can be emitted, then the sparse mapping. Only after that the actual file contents are processed.

This is really rather complex to implement; unlikely without external funding, I guess.

[ThomasWaldmann]

related to #14

[ThomasWaldmann]

see also #5830, basic PAX format support is there, sparse support still TODO.