Data Repo: Replace EGA download client in manifest file generated for 'EGA - Hinxton' repository with new client

[hknahal]

The EgaDemoClient tool appears to be no longer supported by EGA. Instead their documentation (https://ega-archive.org/download/downloader-quickguide-APIv3) points to a new download client called pyega3 (https://github.com/EGA-archive/ega-download-client). The old EgaDemoClient does not work (v 2.2.2) and throws errors, so we will need to update the manifest generation script to use pyega3 instead. Currently in communication with EGA to determine which credentials are required to run the pyega3 download tool.