Add hidden authenticationExtras field to Dataset class

This is needed to add context from the credentialAutofillService
including the final request and other data that the credential provider
may have set. For traditional autofill flows, this will always be null.

Bug:300981001
Test: Built locally & deployed
Unit tests to be added through b/303677885

Change-Id: I49ab64b55ad5a62aa30a6961436263389ea0eb07