Use HDRUK API to retrieve metadata json files

[RayStick]

• Currently, the metadata json file is downloaded manually from the metadata catalogue
• When we get sent the API doc, change the code so that it can retrieve the latest version of the file directly from online
• Feedback to the HDRUK contacts (particularly if there are bugs/improvement) think more widely about how this package feeds into/works alongside the latest HDUK Gateway

[RayStick]

https://api.healthdatagateway.org/api/documentation