Allow tr() import only when Taproot is active

To avoid issues around fund loss, only allow descriptor wallets
to import tr() descriptors after taproot has activated.

src/interfaces/chain.h

@@ -277,6 +277,9 @@ public:
     //! to be prepared to handle this by ignoring notifications about unknown
     //! removed transactions and already added new transactions.
     virtual void requestMempoolTransactions(Notifications& notifications) = 0;
+
+    //! Check if Taproot has activated
+    virtual bool isTaprootActive() const = 0;
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for