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/interfaces/mining.h

@@ -44,16 +44,26 @@ public:
 
     /**
      * Return serialized dummy coinbase transaction.
+     *
+     * @note deprecated: use getCoinbaseTx()
      */
     virtual CTransactionRef getCoinbaseRawTx() = 0;
 
+    /** Return fields needed to construct a coinbase transaction */
+    virtual node::CoinbaseTx getCoinbaseTx() = 0;
+
     /**
      * Return scriptPubKey with SegWit OP_RETURN.
+     *
+     * @note deprecated: use getCoinbaseTx()
      */
     virtual std::vector<unsigned char> getCoinbaseCommitment() = 0;
 
     /**
      * Return which output in the dummy coinbase contains the SegWit OP_RETURN.
+     *
+     * @note deprecated. Scan outputs from getCoinbaseTx() outputs field for the
+     *       SegWit marker.
      */
     virtual int getWitnessCommitmentIndex() = 0;