Add artifact log command

Description

Changes proposed in this merge request:

• Add artifact subcommand group
• Add artifact log command

This merge request, when approved, will close issue/bug: #234 (closed)

CHANGELOG/Release Notes

• Initial WIP version
• Ported to new CASCache and ArtifactCache APIs.

NEWS

+===============
+buildstream 1.4
+===============
+
+  o Added `bst artifact log` subcommand for viewing build logs.
+
 =================
 buildstream 1.3.1
 =================