Handle authentication headers separately

[EmilienM]

We need to revisit authentication headers separately, since we know we may end up talking to different services from different plugins so we need to have a more general solution for someone using fromager to provide credentials.

Originally posted by @dhellmann in #617 (comment)

[dhellmann]

I think it's going to be useful to have a more formal way for users to specify credentials to be used with specific sites. Plugins can do this, but it would be nice if someone didn't have to write an override plugin for a package just to provide credentials. Configuration settings could do it, but then we're asking users to put their API keys in a config file, so I think we want to avoid that. Maybe we want another type of plugin that sets up requests adapters? Or just yields base URLs and credentials so we create the adapters?