Addition of --long option to list-contents

Description

//: This MR includes the addition of the --long option for bst artifact list-contents. When used, it will provide the user with additional information about the contents of the artifact including file type and size.

Changes proposed in this merge request:

• --long option added. An example of what should be be seen when used is:

  bst artifact list-contents --long foo.bst

  foo.bst:
      drwxr-xr-x  dir  1     bar/
      -rw-r--r--  reg  234   bar/foo
      -rwxr-xr-x  exe  109   bar/baz

• Appropriate tests and NEWS entry have been included

//: This MR, once merged, will contribute towards closing #773 (closed)

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