Add support to pass authentication to the downloaders

[alvarolopez]

When downloading data from some repos authentication may be needed. Datahugger should allow to pass a token to be sent in the request headers.

[unkcpz]

Hi @alvarolopez, which data repository you are targeting to? In the new rust implementation at https://github.com/EOSC-Data-Commons/datahugger-ng, we support passing token through the CLI env var. You can find example for github repo download and dyrad dataset download. Please open an issue there if you need more help and we are actively working on that repo.