refactor: Replace string chain name constants with ChainTypes

This commit effectively moves the definition of these constants
out of the chainparamsbase to their own file.

Using the ChainType enums provides better type safety compared to
passing around strings.

The commit is part of an ongoing effort to decouple the libbitcoinkernel
library from the ArgsManager and other functionality that should not be
part of the kernel library.

src/bitcoin-wallet.cpp

@@ -91,7 +91,7 @@ static std::optional<int> WalletAppInit(ArgsManager& args, int argc, char* argv[
         return EXIT_FAILURE;
     }
     // Check for chain settings (Params() calls are only valid after this clause)
-    SelectParams(args.GetChainName());
+    SelectParams(args.GetChainType());
 
     return std::nullopt;
 }