GH-44329: [C++][Flight] Add support for passing pointer options down to transports

cpp/src/arrow/flight/client.h

@@ -117,7 +117,7 @@ struct ARROW_FLIGHT_EXPORT FlightClientOptions {
 
   /// \brief Generic connection options, passed to the underlying
   ///     transport; interpretation is implementation-dependent.
-  std::vector<std::pair<std::string, std::variant<int, std::string>>> generic_options;
+  std::vector<std::pair<std::string, std::variant<int, std::string, void*>>> generic_options;
 
   /// \brief Use TLS without validating the server certificate. Use with caution.
   bool disable_server_verification = false;

cpp/src/arrow/flight/flight_test.cc

@@ -1092,6 +1092,22 @@ TEST_F(TestFlightClient, GenericOptions) {
   ASSERT_THAT(status.message(), ::testing::HasSubstr("resource exhausted"));
 }
 
+TEST_F(TestFlightClient, GenericOptionsPointer) {
+  // WIP(bdm): I want to force GRPC to call my callback here so we can verify
+  // passing down the void* works. This isn't working yet.
+  bool callback_called = false;
+  auto callback = [&callback_called]() { callback_called = true; };
+
+  auto options = FlightClientOptions::Defaults();
+  // TODO(bdm): "some arbitrary function" probably needs to be changed to something
+  // GRPC will call
+  options.generic_options.emplace_back("some arbitrary function", reinterpret_cast<void*>(&callback));
+  ASSERT_OK_AND_ASSIGN(auto location, Location::ForGrpcTcp("localhost", server_->port()));
+  ASSERT_OK_AND_ASSIGN(auto client, FlightClient::Connect(location, options));
+
+  ASSERT_TRUE(callback_called);
+}
+
 TEST_F(TestFlightClient, TimeoutFires) {
   // Server does not exist on this port, so call should fail
   ASSERT_OK_AND_ASSIGN(auto location, Location::ForGrpcTcp("localhost", 30001));

cpp/src/arrow/flight/transport/grpc/grpc_client.cc

@@ -815,6 +815,8 @@ class GrpcClientImpl : public internal::ClientTransport {
         default_args[arg.first] = std::get<int>(arg.second);
       } else if (std::holds_alternative<std::string>(arg.second)) {
         args.SetString(arg.first, std::get<std::string>(arg.second));
+      } else if (std::holds_alternative<void*>(arg.second)) {
+        args.SetPointer(arg.first, std::get<void*>(arg.second));
       }
       // Otherwise unimplemented
     }