Resolve dependencies for files from 3GPP 5G APIs project

-- To be further clarified --

The goal is to resolve references to the 5G_APIs during the validation script in the sa5/MnS project.

Expected behaviour

During the validation script for SA5 / MnS open apis, before executing the validation of individual yaml file,

• fetch and download the content of the all/5G_APIs project in the OpenAPI folder
• the revision to fetch should be TBD