Add Attribution generation pipeline step

[pombredanne]

I would like to have a step at the end of a pipeline that can generate an attribution documentation using AboutCode toolkit.

The basis could be the packages and/or files found on the To side and/or the From or both.
I would like to have a link provided in the UI to download the generated attribution and I would like this to be also available later on as an API operation.

[tdruez]

See https://scancodeio.readthedocs.io/en/latest/custom-pipelines.html#custom-pipeline-example

I would suggest a new Download type, along JSON, XLSX, CycloneDX, SPDX,... instead of a step.

[tdruez]

@pombredanne

pipeline that can generate an attribution documentation using AboutCode toolkit

Can you point me to the documentation (or code) on generating attribution using the Python API?

[tdruez]

Implemented in #712