Activate Taproot/Tapscript on regtest (BIP 341, BIP 342)

Define a versionbits-based activation for the new consensus rules on regtest.
No activation or activation mechanism is defined for testnet or mainnet.

test/functional/rpc_blockchain.py

@@ -146,7 +146,19 @@ class BlockchainTest(BitcoinTestFramework):
                         'possible': True,
                     },
                 },
-                'active': False}
+                'active': False
+            },
+            'taproot': {
+                'type': 'bip9',
+                'bip9': {
+                    'status': 'active',
+                    'start_time': -1,
+                    'timeout': 9223372036854775807,
+                    'since': 0
+                },
+                'height': 0,
+                'active': True
+            }
         })
 
     def _test_getchaintxstats(self):