Add tar/asm.IterateHeaders

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>

go.mod

@@ -6,13 +6,17 @@ require (
 	github.com/fatih/color v1.15.0
 	github.com/magefile/mage v1.14.0
 	github.com/sirupsen/logrus v1.9.0
+	github.com/stretchr/testify v1.9.0
 	github.com/urfave/cli v1.22.12
 )
 
 require (
 	github.com/cpuguy83/go-md2man/v2 v2.0.2 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.17 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	golang.org/x/sys v0.6.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )