wallet, rpc: Add gethdkeys RPC

gethdkeys retrieves all HD keys stored in the wallet's descriptors.

src/rpc/client.cpp

@@ -275,6 +275,9 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "logging", 1, "exclude" },
     { "disconnectnode", 1, "nodeid" },
     { "upgradewallet", 0, "version" },
+    { "gethdkeys", 0, "active_only" },
+    { "gethdkeys", 0, "options" },
+    { "gethdkeys", 0, "private" },
     // Echo with conversion (For testing only)
     { "echojson", 0, "arg0" },
     { "echojson", 1, "arg1" },