fetch api: add support for downloading raw response (#4917)
* Factor out `BaseRequestOpts`
... to make it easier to find the docs from methods that use it.
* fetch api: add support for downloading raw response
I need to make an authenticated request to the media repo, and expect to get a
binary file back. AFAICT there is no easy way to do that right now.
* Clarify doc strings
* Various fixes