Introduce mgp_trans api (#175)

* Added public interface for registering mgp_trans and extended modules accordingly

* Added test for mgp_trans

* Added mg.transformations() to the module registry

Co-authored-by: János Benjamin Antal <antaljanosbenjamin@users.noreply.github.com>

tests/unit/mgp_trans_c_api.cpp

@@ -0,0 +1,24 @@
+#include "gtest/gtest.h"
+
+#include "query/procedure/mg_procedure_impl.hpp"
+#include "query/procedure/module.hpp"
+#include "test_utils.hpp"
+
+TEST(MgpTransTest, TestMgpTransApi) {
+  constexpr auto no_op_cb = [](const mgp_messages *msg, mgp_graph *graph, mgp_result *result, mgp_memory *memory) {};
+  mgp_module module(utils::NewDeleteResource());
+  // If this is false, then mgp_module_add_transformation()
+  // correctly calls IsValidIdentifier(). We don't need to test
+  // for different string cases as these are all handled by
+  // IsValidIdentifier().
+  // Maybe add a mock instead and expect IsValidIdentifier() to be called once?
+  EXPECT_FALSE(mgp_module_add_transformation(&module, "dash-dash", no_op_cb));
+  EXPECT_TRUE(module.transformations.size() == 0);
+
+  EXPECT_TRUE(mgp_module_add_transformation(&module, "transform", no_op_cb));
+  EXPECT_NE(module.transformations.find("transform"), module.transformations.end());
+
+  // Try to register a transformation twice
+  EXPECT_FALSE(mgp_module_add_transformation(&module, "transform", no_op_cb));
+  EXPECT_TRUE(module.transformations.size() == 1);
+}