This allows reading the metadata contained in tar-split
without expensively recreating the whole tar stream
including full contents.

We have two use cases for this:
- In a situation where tar-split is distributed along with
  a separate metadata stream, ensuring that the two are
  exactly consistent
- Reading the tar headers allows making a ~cheap check
  of consistency of on-disk layers, just checking that the
  files exist in expected sizes, without reading the full
  contents.

This can be implemented outside of this repo, but it's
not ideal:
- The function necessarily hard-codes some assumptions
  about how tar-split determines the boundaries of
  SegmentType/FileType entries (or, indeed, whether it
  uses FileType entries at all). That's best maintained
  directly beside the code that creates this.
- The ExpectedPadding() value is not currently exported,
  so the consumer would have to heuristically guess where
  the padding ends.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>