Addition of bst artifact list-contents

Description

//: # This MR includes the addition of a bst artifact list contents subcommand and a suitable test for this feature.

Changes proposed in this merge request:

• bst artifact list-contents command which can be used with either element name, artifact ref or glob expresssion
• Test to ensure it works as it should

To show what the output should be, an element name such as:

bst artifact list-contents bar.bst

gives

bar.bst:
        foo 
        foo/bar

and an artifact ref/glob expression such as:

bst artifact list-contents foo/bar/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

gives

foo/bar/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:
        foo 
        foo/bar       

This merge request partially addresses: #773 (closed)

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