Well it does look like Annatar has already maybe approached Celebrimbor, it's a shame we didn't get to see it, maybe in flash back? I think we might not find out till the end of this series, I think it could be HalbrandWell I dont think its Sauron. Sauron reappears as Annatar, friend to the elves who he ultimately betrays.
My guess is its not Gandalf either.
I don't believe there is anything in the source material to suggest who it is.
Henry Kissinger?