Update local execute function (#2)

* Add local ExecuteFunction to repo

* Change FunctionExecutionContext to FunctionExecutionContextInternal

* Fix typo

* Renames in TitleAuthentication

* Revisit execute function to account for latest mainserver changes

* Use a single static http client instead of a few instances

* Change ExecutionTimeSeconds to int instead of double

* Move ExecuteFunction to csharp directory

* ExecutionTimeSeconds -> ExecutionTimeMilliseconds