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/bench/load_external.cpp

@@ -6,6 +6,7 @@
 #include <bench/data.h>
 #include <chainparams.h>
 #include <test/util/setup_common.h>
+#include <util/chaintype.h>
 #include <validation.h>
 
 /**
@@ -22,7 +23,7 @@
  */
 static void LoadExternalBlockFile(benchmark::Bench& bench)
 {
-    const auto testing_setup{MakeNoLogFileContext<const TestingSetup>(CBaseChainParams::MAIN)};
+    const auto testing_setup{MakeNoLogFileContext<const TestingSetup>(ChainType::MAIN)};
 
     // Create a single block as in the blocks files (magic bytes, block size,
     // block data) as a stream object.