PBS AMP endpoint support for GPP

[bretg]

AMP has added support for GPP consent as described in https://github.com/ampproject/amphtml/issues/38628

The design is that RTC vendor strings will be enhanced to pass one more additional value:

&gpp_sid=CONSENT_METADATA(gppSectionId)

In addition to this new field, two existing fields can take new values: if the consent_type field is 4, then the consent_string value is formatted as a GPP string.

So basically this issue an extension to the original AMP consent issue https://github.com/prebid/prebid-server/issues/1817

1. If consent_type==4
   1. if the consent_string query string parameter is present, copy the the value as a string to ORTB regs.gpp.
   2. if the gpp_sid query string parameter is present, check that the value is a comma-separated list of numbers and if so, copy the value as a string to ORTB regs.gpp_sid. If the validation fails, regs.gpp_sid should be empty.
   3. If in debug mode, any URL decode or validation issue should be emitted as a warning.

Note: GPP strings are supposed to be URL safe, so no decoding should be necessary.

[bretg]

FYI - the rubicon callout-vendor file update at https://github.com/ampproject/amphtml/pull/39906