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/rest.cpp

@@ -273,6 +273,9 @@ static bool rest_block_notxdetails(HTTPRequest* req, const std::string& strURIPa
     return rest_block(req, strURIPart, false);
 }
 
+// A bit of a hack - dependency on a function defined in rpc/blockchain.cpp
+UniValue getblockchaininfo(const UniValue& params, bool fHelp);
+
 static bool rest_chaininfo(HTTPRequest* req, const std::string& strURIPart)
 {
     if (!CheckWarmup(req))