Move getMessage and getMessageFromRegistry to cpp and add test

This change moves getMessage and getMessageFromRegistry to a .cpp file
so that they can be easily tested.

Tested: Unit test passes
Signed-off-by: Sui Chen <suichen@google.com>
Change-Id: Ia9fc91e5a47036198bf013ff3ea21ea9f6d5259a
diff --git a/redfish-core/include/registries.hpp b/redfish-core/include/registries.hpp
index 03e8bc5..bad31c4 100644
--- a/redfish-core/include/registries.hpp
+++ b/redfish-core/include/registries.hpp
@@ -120,4 +120,9 @@
     return response;
 }
 
+const Message* getMessage(std::string_view messageID);
+
+const Message* getMessageFromRegistry(const std::string& messageKey,
+                                      std::span<const MessageEntry> registry);
+
 } // namespace redfish::registries