multiprocess: Add interfaces::Node::broadCastTransaction method

This fixes a null pointer crash in the bitcoin-gui PSBT dialog. The
bitcoin-gui interfaces::Node object has a null NodeContext pointer, and
can't broadcast transactions directly. It needs to broadcast
transactions through the bitcoin-node process instead.

src/node/interfaces.cpp

@@ -261,6 +261,10 @@ public:
         LOCK(::cs_main);
         return chainman().ActiveChainstate().CoinsTip().GetCoin(output, coin);
     }
+    TransactionError broadcastTransaction(CTransactionRef tx, CAmount max_tx_fee, std::string& err_string) override
+    {
+        return BroadcastTransaction(*m_context, std::move(tx), err_string, max_tx_fee, /*relay=*/ true, /*wait_callback=*/ false);
+    }
     WalletClient& walletClient() override
     {
         return *Assert(m_context->wallet_client);