rpc: Register calls where they are defined

Split out methods to every module, apart from 'help' and 'stop' which
are implemented in rpcserver.cpp itself.

- This makes it easier to add or remove RPC commands - no longer everything that includes
    rpcserver.h has to be rebuilt when there's a change there.
- Cleans up `rpc/server.h` by getting rid of the huge cluttered list of function definitions.
- Removes most of the bitcoin-specific code from rpcserver.cpp and .h.

Continues #7307 for the non-wallet.

src/init.cpp

@@ -24,6 +24,7 @@
 #include "net.h"
 #include "policy/policy.h"
 #include "rpc/server.h"
+#include "rpc/register.h"
 #include "script/standard.h"
 #include "script/sigcache.h"
 #include "scheduler.h"
@@ -913,10 +914,11 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
         fPruneMode = true;
     }
 
+    RegisterAllCoreRPCCommands(tableRPC);
 #ifdef ENABLE_WALLET
     bool fDisableWallet = GetBoolArg("-disablewallet", false);
     if (!fDisableWallet)
-        walletRegisterRPCCommands();
+        RegisterWalletRPCCommands(tableRPC);
 #endif
 
     nConnectTimeout = GetArg("-timeout", DEFAULT_CONNECT_TIMEOUT);