It is very useful to be able to add extra PAX headers to tar entries. For example, a tar file containing maven artifacts could have extra headers carrying the groupid, artifactid, version, classifier, etc.
If the appropriate prefixes are used, these headers can be extracted to posix extended attributes by gnu and bsdtar.
This change requires adding a map to TarArchiveEntry to carry the extra headers, plus modifications to the TarArchive*Stream to save unrecognized headers when reading, and to add any extra headers when writing.
I have created a prototype implementation, but have not merged it into my fork of the project. I don't have full tests written, because I was using gnutar as an oracle.
I have also ignored the issue of writing values to standard headers like size, though since the PAX specification states that doing things like setting size=100 (if the real size is not in fact 100) is undefined, so I'm technically in compliance. The temptation is to do what was asked, then on close pop up a "Were you sure?" dialog, but that's mean. I guess I could use this to set the appropriate entry fields if doing so makes sense, but the easiest approach is to block setting any headers that would be consumed by the tar implementation when reading.