Support for GPC

[patmmccann]

We should set ortb2.regs.ext.gpc to 1 if GPC headers indicate the user has opted out, following #6270 (comment)

Some guidance suggests the third character of the US Privacy string should be set to 'Y'. Publishers or bid adapters are free to modify the string set by the cmp to effectuate this or entities may choose to resolve conflicts server side as suggested in InteractiveAdvertisingBureau/openrtb#98

[patmmccann]

@AramZS fyi