Add ability to request contents of final web page

For some authentication methods the token is not passed in the URL but is returned inside the response for the final URL. This change adds the possibility to retrieve that content so that it can be parsed to process any authentication data.

In its current form this will work only with the in-process web browser. Supporting this in the remote-process browser would require more work.

An accompanying change in signond headers is also needed.

This enhancement is needed to support Cisco Webex authentication using an additional plugin.