node: change a tx-relay on/off flag to enum

Previously the `bool relay` argument to `BroadcastTransaction()`
designated:

```
relay=true: add to the mempool and broadcast to all peers
relay=false: add to the mempool
```

Change this to an `enum`, so it is more readable and easier to extend
with a 3rd option. Consider these example call sites:

```cpp
Paint(true);
// Or
Paint(/*is_red=*/true);
```

vs

```cpp
Paint(RED);
```

The idea for putting `TxBroadcastMethod` into `node/types.h` by Ryan.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/interfaces.cpp

@@ -363,7 +363,12 @@ public:
     }
     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);
+        return BroadcastTransaction(*m_context,
+                                    std::move(tx),
+                                    err_string,
+                                    max_tx_fee,
+                                    TxBroadcast::MEMPOOL_AND_BROADCAST_TO_ALL,
+                                    /*wait_callback=*/false);
     }
     WalletLoader& walletLoader() override
     {
@@ -672,10 +677,10 @@ public:
     }
     bool broadcastTransaction(const CTransactionRef& tx,
         const CAmount& max_tx_fee,
-        bool relay,
+        TxBroadcast broadcast_method,
         std::string& err_string) override
     {
-        const TransactionError err = BroadcastTransaction(m_node, tx, err_string, max_tx_fee, relay, /*wait_callback=*/false);
+        const TransactionError err = BroadcastTransaction(m_node, tx, err_string, max_tx_fee, broadcast_method, /*wait_callback=*/false);
         // Chain clients only care about failures to accept the tx to the mempool. Disregard non-mempool related failures.
         // Note: this will need to be updated if BroadcastTransactions() is updated to return other non-mempool failures
         // that Chain clients do not need to know about.