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/rpc/blockchain.cpp

@@ -911,3 +911,31 @@ UniValue reconsiderblock(const UniValue& params, bool fHelp)
 
     return NullUniValue;
 }
+
+static const CRPCCommand commands[] =
+{ //  category              name                      actor (function)         okSafeMode
+  //  --------------------- ------------------------  -----------------------  ----------
+    { "blockchain",         "getblockchaininfo",      &getblockchaininfo,      true  },
+    { "blockchain",         "getbestblockhash",       &getbestblockhash,       true  },
+    { "blockchain",         "getblockcount",          &getblockcount,          true  },
+    { "blockchain",         "getblock",               &getblock,               true  },
+    { "blockchain",         "getblockhash",           &getblockhash,           true  },
+    { "blockchain",         "getblockheader",         &getblockheader,         true  },
+    { "blockchain",         "getchaintips",           &getchaintips,           true  },
+    { "blockchain",         "getdifficulty",          &getdifficulty,          true  },
+    { "blockchain",         "getmempoolinfo",         &getmempoolinfo,         true  },
+    { "blockchain",         "getrawmempool",          &getrawmempool,          true  },
+    { "blockchain",         "gettxout",               &gettxout,               true  },
+    { "blockchain",         "gettxoutsetinfo",        &gettxoutsetinfo,        true  },
+    { "blockchain",         "verifychain",            &verifychain,            true  },
+
+    /* Not shown in help */
+    { "hidden",             "invalidateblock",        &invalidateblock,        true  },
+    { "hidden",             "reconsiderblock",        &reconsiderblock,        true  },
+};
+
+void RegisterBlockchainRPCCommands(CRPCTable &tableRPC)
+{
+    for (unsigned int vcidx = 0; vcidx < ARRAYLEN(commands); vcidx++)
+        tableRPC.appendCommand(commands[vcidx].name, &commands[vcidx]);
+}