Possibility to add custom headers for HTTP downloads

**Describe the problem/challenge you have**
Some websites request to add specific HTTP headers, like for example API Token to access the files.
Currently it seems not possible to add those headers in the yaml file.
Example: wget -O --header="Authorization: Token *************"

**Describe the solution you'd like**
Add a new field in the vendir.yaml configuration file for each header.

**Anything else you would like to add:**
Thank you :)

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