Reduce multiple oem_ibm entry points in pldmd

Abstract the custom method of `OEM-IBM` into the oem-ibm.hpp file
to prevent the continuous increase of custom code and reduce multiple
`OEM-IBM` entry points in pldmd.

Tested: enabled oem-ibm and built pldm successfully.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ieddb8d12281553e70bdb1c333bd29425c9d14fb0
diff --git a/libpldmresponder/test/libpldmresponder_fru_test.cpp b/libpldmresponder/test/libpldmresponder_fru_test.cpp
index 2d7b7ec..d654fca 100644
--- a/libpldmresponder/test/libpldmresponder_fru_test.cpp
+++ b/libpldmresponder/test/libpldmresponder_fru_test.cpp
@@ -117,7 +117,7 @@
 
     pldm::responder::FruImpl mockedFruHandler(
         FRU_JSONS_DIR, "./fru_jsons/fru_master/fru_master.json", pdrRepo.get(),
-        entityTree.get(), bmcEntityTree.get(), nullptr);
+        entityTree.get(), bmcEntityTree.get());
 
     pldm_entity systemEntity{0x2d01, 1, 0};
     pldm_entity chassisEntity{0x2d, 1, 1};
@@ -170,7 +170,7 @@
     InterfaceMap iface = {{"xyz.openbmc_project.Inventory.Item.Chassis", {}}};
     pldm::responder::FruImpl mockedFruHandler(
         FRU_JSONS_DIR, "./fru_jsons/fru_master/fru_master.json", pdrRepo.get(),
-        entityTree.get(), bmcEntityTree.get(), nullptr);
+        entityTree.get(), bmcEntityTree.get());
 
     // Good path
     auto entityPtr = mockedFruHandler.getEntityByObjectPath(iface);