ScenarioManager - Review and cleanup code around `add_file_as_data_asset`

[victorterpstra]

Review the code in the ScenarioManager around registering a file as an asset in CPD.
Over the lifetime of DSX/ICPd/CPP/CPDaaS the necessary steps and APIs have been changing.
There have been major fixes, but there is deprecated code that is confusing.

Potential problem:
In the past, most methods around this topic have been static and could have been called directly.
Given that CPDaaS requires the project, is was getting harder to keep maintaining static methods.
Code cleanup may cause backward incompatibilities.