Add a delegate method for HTTP authentication and a related
`AuthenticationRequest` object that carries with it the URL as well as
whether or not the authentication request is for a proxy or not.
This is now separate from the prompt API because requesting
authentication doesn't necessarily involve prompting -- this is an
implementation detail of the embedder. In addition, the internal bits
are cleaned up slightly.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>