Feature request: support basic (or custom headers) auth for public gateway requests

It would be good if ipfs companion would allow me to use gateways which are running behind nginx with basic auth on them. The idea is a username/pw fields next to the public gateway configuration.

An alternative which works too would be to allow users to provide custom gateway request headers.

Start by tracing how the public gateway configuration is stored and how gateway requests are made. Decide whether the completed change supports username/password fields, custom request headers, or both, and verify that configured credentials are applied to requests sent to authenticated gateways.

Written by the indexing model from the issue text.