Handle GPID (openrtb endpoint)

[epechuzal]

Sister PR to #1107, this time for openRTB endpoint.

From a talk with STX they want the param to be in imp.ext.gpid, so I added it there.

[sjfields100]

Hey guys, looks like this update may have broken sharethrough on our site-- it doesn't work above prebid v 5.13. Did any required configurations change for sharethrough or is it just the pkey that is still required? It works below v 5.13.

Here is a test page:
https://wishlist.indy100.com/37-gadgets-that-make-cleaning-10x-easier/?test=cleanup&pbjs_debug=true&5.13.0

Sceenshot of errors in network tab of DOM:

[epechuzal]

Hey guys, looks like this update may have broken sharethrough on our site-- it doesn't work above prebid v 5.13. Did any required configurations change for sharethrough or is it just the pkey that is still required? It works below v 5.13.

Required configuration should not have changed. I do not get HTTP 400 responses when I visit your test page either.