Change binding to RxNorm to extensible to accommodate new drugs

From Courtney Bland (cnbland@aetna.com) via [zulip]( https://chat.fhir.org/#narrow/stream/197730-Da-Vinci.20PDex.20Drug.20Formulary/topic/NDC.2C.20RXCUI.20and.20RXNorm )

"There are times that we add drugs to our formulary before an RXCUI is created. When the IG indicates that we must return back an RxNORM ID, we are interpreting that to mean an RXCUI. For those cases in which we have added a drug prior to the creation of an RxCUI, we were planning to report back the NDC. There are two problems with this approach.

Version 1.0.1 makes RxNorm required instead of extensible. What is the recommended approach for these drugs that don't have an RXCui?"

 In US Core Medication, the binding to the same Value Set is 'extensible'. Shouldn't it be extensible here to deal with drugs that do not yet have an RxNorm code?