Support loading shared libraries for custom operators

- Add a new command line option to allow users to specify a custom
  defined dll library
- Add a custom registry to store all registered libraries
- Add a dummy example (custom_op_example.cpp) for demonstrating this new
  feature

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: I7c360835933f77e33fcbd772cabfe01d82282d47
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6556b27..117bc9f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,3 +20,5 @@
 if(TOSA_TOOLS_BUILD_REFERENCE_MODEL)
     add_subdirectory(reference_model)
 endif()
+
+add_subdirectory(reference_model/custom_op_example)