Looks up an offer by its third-party store reference (e.g. a Google Play or Apple App Store product ID), not by Magine's internal offerId. The provider name is case-insensitive (e.g. 'google', 'GOOGLE').

Known limitation: this lookup currently returns 404 thirdPartyOfferNotFound for offers with thirdPartyOfferSubtype: pass, even when the externalOfferId is valid and the offer exists. It has only been confirmed working for subscription-subtype third-party offers.