Add GPP Support to AMP syncs

[patmmccann]

Is your feature request related to a problem? Please describe.
User syncs don't pass gpp strings

Describe the solution you'd like
Gpp strings added to https://github.com/prebid/prebid-universal-creative/blob/master/src/cookieSync.js

[patmmccann]

blocked by ampproject/amphtml#39769

[patmmccann]

unvblocked by ampproject/amphtml#38628 (comment)

[patmmccann]

@nicgallardo are you aware of an amp page with a gpp string on it?