Support raster.json via proxy

[keller-mark]

Currently, when using the widget via proxy (by running vc.widget(proxy=True)), the url field of each file definition is updated so that includes the absolute URL of the remote server (code here).

However, for raster.json, the URLs may be located in the options, and then 2 issues occur:

• the widget tries to add both url and options to the file definition, creating an invalid config
• the url fields within options do not get updated

To resolve this, the widget.js code should check for raster.json with options and then update the inner url fields