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/types.h

@@ -16,10 +16,13 @@
 #include <consensus/amount.h>
 #include <cstddef>
 #include <cstdint>
+#include <optional>
 #include <policy/policy.h>
+#include <primitives/transaction.h>
 #include <script/script.h>
 #include <uint256.h>
 #include <util/time.h>
+#include <vector>
 
 namespace node {
 enum class TransactionError {
@@ -99,6 +102,53 @@ struct BlockCheckOptions {
     bool check_pow{true};
 };
 
+/**
+ * Template containing all coinbase transaction fields that are set by our
+ * miner code. Clients are expected to add their own outputs and typically
+ * also expand the scriptSig.
+ */
+struct CoinbaseTx {
+    /* nVersion */
+    uint32_t version;
+    /* nSequence for the only coinbase transaction input */
+    uint32_t sequence;
+    /**
+     * Prefix which needs to be placed at the beginning of the scriptSig.
+     * Clients may append extra data to this as long as the overall scriptSig
+     * size is 100 bytes or less, to avoid the block being rejected with
+     * "bad-cb-length" error.
+     *
+     * Currently with BIP 34, the prefix is guaranteed to be less than 8 bytes,
+     * but future soft forks could require longer prefixes.
+     */
+    CScript script_sig_prefix;
+    /**
+     * The first (and only) witness stack element of the coinbase input.
+     *
+     * Omitted for block templates without witness data.
+     *
+     * This is currently the BIP 141 witness reserved value, and can be chosen
+     * arbitrarily by the node, but future soft forks may constrain it.
+     */
+    std::optional<uint256> witness;
+    /**
+     * Block subsidy plus fees, minus any non-zero required_outputs.
+     *
+     * Currently there are no non-zero required_outputs, so block_reward_remaining
+     * is the entire block reward. See also required_outputs.
+     */
+    CAmount block_reward_remaining;
+    /*
+     * To be included as the last outputs in the coinbase transaction.
+     * Currently this is only the witness commitment OP_RETURN, but future
+     * softforks or a custom mining patch could add more.
+     *
+     * The dummy output that spends the full reward is excluded.
+     */
+    std::vector<CTxOut> required_outputs;
+    uint32_t lock_time;
+};
+
 /**
  * How to broadcast a local transaction.
  * Used to influence `BroadcastTransaction()` and its callers.