Add fragment reference from_image command

[DomAyre]

Why

Allows the user to generate the fragment reference rego for building policies and insert that in the final policy.

How

• Add a command to generate the fragment reference
• Add the --with-fragments flag to acipolicygen
• Add testing

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• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally? (pip install wheel==0.30.0 required)
• My extension version conforms to the Extension version schema