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/checkblock.cpp

@@ -9,6 +9,7 @@
 #include <common/args.h>
 #include <consensus/validation.h>
 #include <streams.h>
+#include <util/chaintype.h>
 #include <validation.h>
 
 // These are the two major time-sinks which happen after we have fully received
@@ -36,7 +37,7 @@ static void DeserializeAndCheckBlockTest(benchmark::Bench& bench)
     stream.write({&a, 1}); // Prevent compaction
 
     ArgsManager bench_args;
-    const auto chainParams = CreateChainParams(bench_args, CBaseChainParams::MAIN);
+    const auto chainParams = CreateChainParams(bench_args, ChainType::MAIN);
 
     bench.unit("block").run([&] {
         CBlock block; // Note that CBlock caches its checked state, so we need to recreate it here