Fixes callbacks causing issues with IL2CPP and marshalling delegates when searching steam server list

[westonleavitt]

Have not tested this extensively, but this fixes the error I reported of #700 regarding IL2CPP and the Marshalling delegates issues when searching the steam server list. It is currently functioning now for our use case so providing as a help for community.

Changes made: Adjusted for use with IL2CPP delegate marshaling --Delegates are static, decorated with MonoPInvokeCallback and Instance methods are mapped via a static dictionary