WIP: Add OCI element plugin

Description

Pending tasks:

• Fix RootfsBlob.__init__ to use a temporary directory for staging
• Allow users to specify labels
• Handle architecture and os fields in a sensible way
• Fixup documentation
• See if we can run "real" integration tests, using runc, docker or something similar

Changes proposed in this merge request:

• Add OCI element plugin

This merge request, when approved, will close issue/bug: #349 (moved)

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