Support authentication in import server

[bpauli]

At the moment, it is not possible to add a custom header with cookie information to authenticate against a page that should be imported because the HelixImportServer removes the header.cookie information in [1]

It would be nice to have the possibility to add a custom header with cookie information to authenticate.

[1] https://github.com/adobe/helix-cli/blob/main/src/server/HelixImportServer.js#L99