ApiSandbox: Allow displaying query parameters in various formats

ApiSandbox: Allow displaying query parameters in various formats

I9c24a230 added a box with a JSON-formatted version of the request
parmeters, saying "In the future we can provide more formats via a
dropdown". That future should have been then. It is now.

Beyond simply making it possible to select the format via a dropdown,
this change adds a JS hook, apisandbox.formatRequest, to allow adding
new formats from extensions, gadgets, and site scripts instead of
requiring them all to be added to core.

Change-Id: I20b093e0891f36cb3f1faa3eb3d6aed0f17b6b7d