[bitcoin/bitcoin] Refactoring CRPCCommand with enum category (#13945)

isghe commented on this pull request.

> @@ -131,10 +133,10 @@ typedef UniValue(*rpcfn_type)(const JSONRPCRequest& jsonRequest);
class CRPCCommand
{
public:

done in 7f997989bc63fe14ada94094751c8b4551a4178c, but consider that usually, old style C++ developer prefer `class public` to ‘struct’ for portability (struct forward declarations were a little bit tricky). In the same file `server.h` there are others `class public`