support ustar format

HI,

Feature description (ustar)

I don't know which formats can be configured, but the maven-assembly-maven-plugin uses plexus-compress which in turn uses commons-compress... and there I can chose between:

plexus-archiver/src/main/java/org/codehaus/plexus/archiver/tar/TarLongFileMode.java at d890260b9c960930f6a846de4952b9696f87454d · codehaus-plexus/plexus-archiver (github.com)

public enum TarLongFileMode {
    warn,
    fail,
    truncate,
    gnu,
    omit,
    posix,
    posix_warn;
}

I do not know how to create ustar archives. Maybe omit or truncate will result in a v7 or ustar archive, I really do not know. But looking at the constants, there is only:

TarArchiveOutputStream (Apache Commons Compress 1.26.2 API)

GNU, POSIX, TRUNCATE, ERROR

Use case 

bsdtar (used by Mac and AIX, for example, as well as OpenBSD and maybe some obsolete systems like HaikuOS, HPUX, Solaris) do not support GNU nor Posix extensions. They all support v7, they all support the ustar extensions.

ustar is therefore the only suitable format at the moment (Note: Not entirely true. AIX will implement posix/pax in AIX 7.3.1, current Mac can now also do pax). However, AIX 7.3.1 is relatively new, and there may be a lot of old systems which will justify implementing the ustar format