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/zcbenchmarks.cpp

@@ -461,6 +461,8 @@ double benchmark_loadwallet()
     return res;
 }
 
+extern UniValue listunspent(const UniValue& params, bool fHelp);
+
 double benchmark_listunspent()
 {
     UniValue params(UniValue::VARR);