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/interfaces/chain.h

@@ -7,6 +7,7 @@
 
 #include <blockfilter.h>
 #include <common/settings.h>
+#include <node/types.h>
 #include <primitives/transaction.h>
 #include <util/result.h>
 
@@ -206,13 +207,19 @@ public:
     //! Check if transaction has descendants in mempool.
     virtual bool hasDescendantsInMempool(const Txid& txid) = 0;
 
-    //! Transaction is added to memory pool, if the transaction fee is below the
-    //! amount specified by max_tx_fee, and broadcast to all peers if relay is set to true.
-    //! Return false if the transaction could not be added due to the fee or for another reason.
+    //! Process a local transaction, optionally adding it to the mempool and
+    //! optionally broadcasting it to the network.
+    //! @param[in] tx Transaction to process.
+    //! @param[in] max_tx_fee Don't add the transaction to the mempool or
+    //! broadcast it if its fee is higher than this.
+    //! @param[in] broadcast_method Whether to add the transaction to the
+    //! mempool and how/whether to broadcast it.
+    //! @param[out] err_string Set if an error occurs.
+    //! @return False if the transaction could not be added due to the fee or for another reason.
     virtual bool broadcastTransaction(const CTransactionRef& tx,
-        const CAmount& max_tx_fee,
-        bool relay,
-        std::string& err_string) = 0;
+                                      const CAmount& max_tx_fee,
+                                      node::TxBroadcast broadcast_method,
+                                      std::string& err_string) = 0;
 
     //! Calculate mempool ancestor and descendant counts for the given transaction.
     virtual void getTransactionAncestry(const Txid& txid, size_t& ancestors, size_t& descendants, size_t* ancestorsize = nullptr, CAmount* ancestorfees = nullptr) = 0;