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/wallet/rpcwallet.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_WALLET_RPCWALLET_H
 #define BITCOIN_WALLET_RPCWALLET_H
 
-void walletRegisterRPCCommands();
+class CRPCTable;
+
+void RegisterWalletRPCCommands(CRPCTable &tableRPC);
 
 #endif //BITCOIN_WALLET_RPCWALLET_H