Add plugin api to add native dll searching path (#343)

Add plugin api to add native dll searching path (#343)

There are some cases to add dll and native library searching path through plugin API.
The adding dll searching path was already supported, but the adding native library searching part is missing, so this part is added.

The added searching path by plugin has the highest priority. (VD requirement)

Additionally, an environment for building simple libdotnet_plugin.so for testing was also added.