mining: add new getCoinbaseTx() returning a struct

Introduce a new method intended to replace getCoinbaseRawTx(), which
provides a struct with everything clients need to construct a coinbase.
This is safer than providing a raw dummy coinbase that clients then have
to manipulate.

The CoinbaseTx data is populated during the dummy transaction generation
and stored in struct CBlockTemplate.

Expand the interface_ipc.py functional test to document its usage
and ensure equivalence.

src/node/interfaces.cpp

@@ -84,6 +84,7 @@ using interfaces::WalletLoader;
 using kernel::ChainstateRole;
 using node::BlockAssembler;
 using node::BlockWaitOptions;
+using node::CoinbaseTx;
 using util::Join;
 
 namespace node {
@@ -893,6 +894,11 @@ public:
         return m_block_template->block.vtx[0];
     }
 
+    CoinbaseTx getCoinbaseTx() override
+    {
+        return m_block_template->m_coinbase_tx;
+    }
+
     std::vector<unsigned char> getCoinbaseCommitment() override
     {
         return m_block_template->vchCoinbaseCommitment;