[miner] omit dummy extraNonce via IPC

Previously the coinbase transaction generated by our miner code was
not used downstream, because the getblocktemplate RPC excludes it.

Since the Mining IPC interface was introduced in #30200 we do expose
this dummy coinbase transaction. In Stratum v2 several parts of it
are communicated downstream, including the scriptSig.

This commit removes the dummy extraNonce from the coinbase scriptSig
in block templates requested via IPC. This limits the scriptSig
to what is essential for consensus (BIP34) and removes the need for
external mining software to remove the dummy, or even ignore
the scriptSig we provide and generate it some other way. This
could cause problems if a future soft fork requires additional
data to be committed here.

A test is added to verify the new IPC behavior.

It achieves this by introducing an include_dummy_extranonce
option which defaults to false with all test code updated to
set it to true. Because this option is not exposed via IPC,
callers will no longer see it.

The caller needs to ensure that for blocks 1 through 16
they pad the scriptSig in order to avoid bad-cb-length.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

test/functional/interface_ipc.py

@@ -20,10 +20,14 @@ from test_framework.messages import (
     ser_uint256,
     COIN,
 )
+from test_framework.script import (
+    CScript,
+    CScriptNum,
+)
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
-    assert_not_equal
+    assert_not_equal,
 )
 from test_framework.wallet import MiniWallet
 from typing import Optional
@@ -194,6 +198,12 @@ class IPCInterfaceTest(BitcoinTestFramework):
         coinbase_tx.vin = [CTxIn()]
         coinbase_tx.vin[0].prevout = NULL_OUTPOINT
         coinbase_tx.vin[0].nSequence = coinbase_res.sequence
+
+        # Verify there's no dummy extraNonce in the coinbase scriptSig
+        current_block_height = self.nodes[0].getchaintips()[0]["height"]
+        expected_scriptsig = CScript([CScriptNum(current_block_height + 1)])
+        assert_equal(coinbase_res.scriptSigPrefix.hex(), expected_scriptsig.hex())
+
         # Typically a mining pool appends its name and an extraNonce
         coinbase_tx.vin[0].scriptSig = coinbase_res.scriptSigPrefix