Specify headers for feed

[kuznetsss]

• I have read this document: https://miniflux.app/opinionated.html#feature-request

Hi, thank for the great tool!

I would like to subscribe to some feeds which requires authorization by providing X-Service-Token header. So it would be nice to be able to specify headers for each feed in the same way as cookies now.

If maintainers would approve it as a useful feature I could try to make PR by myself. It looks doable since RequestBuilder already has method WithHeader(). Just will need to add headers field to model, to ui and probably to db migration.