Merge bitcoin-core#771: Support testnet4 chain

f0796e4 Support testnet4 chain (Roman Zeyde)

Pull request description:

  Otherwise, HWI fails with the following error when used as a external signer for Bitcoin Core 28.0:

      {"error": "hwi.py: error: argument --chain: invalid choice: 'testnet4' (choose from main, test, regtest, signet)", "code": -2}

ACKs for top commit:
  achow101:
    ACK f0796e4

Tree-SHA512: cd819e47610eeaefbfb3eb3fa2335d035fdea7c7187c25eb16c483c7d94b59bb9ecd382de69e9fbceaf8c7bede6931b6f16b5f765a6dbe4094eda1bbed6ff4bb

Author: achow101

hwilib/common.py

@@ -18,6 +18,7 @@ class Chain(Enum):
     TEST = 1 #: Bitcoin Test network
     REGTEST = 2 #: Bitcoin Core Regression Test network
     SIGNET = 3 #: Bitcoin Signet
+    TESTNET4 = 4 #: Bitcoin Test network
 
     def __str__(self) -> str:
         return str(self.name).lower()