Adds Network::to_core_arg

This allows users to create `Network` instances from `bitcoind -chain` values

bitcoin/src/network/constants.rs

@@ -119,6 +119,28 @@ impl Network {
             Network::Regtest => "regtest",
         }
     }
+
+    /// Converts a `bitcoind -chain` argument name to its equivalent `Network`.
+    ///
+    /// ```bash
+    /// $ bitcoin-23.0/bin/bitcoind --help | grep -C 3 '\-chain=<chain>'
+    /// Chain selection options:
+    ///
+    /// -chain=<chain>
+    /// Use the chain <chain> (default: main). Allowed values: main, test, signet, regtest
+    /// ```
+    pub fn from_core_arg(core_arg: &str) -> Result<Self, ParseNetworkError> {
+        use Network::*;
+
+        let network = match core_arg {
+           "main" => Bitcoin,
+           "test" => Testnet,
+           "signet" => Signet, 
+           "regtest" => Regtest,
+           _ => return Err(ParseNetworkError(core_arg.to_owned())),
+        };
+        Ok(network)
+    }
 }
 
 /// An error in parsing network string.