The Cognito Identity Provider for unity is not distributed as a part of the aws sdk for dotnet. As a means to more easily get the Cognito-stuff into your Unity project, instead of having to do the workaround all by yourself of porting the .NET 4.5 Cognito Identity Provider to .NET 2.0, I did it for you and provide the code and dll here.

Of course this is all obsolete with Unity 2018, as it will support .NET 4.6, wherein you can use the default AWS SDK for .NET 4.5. But for all other projects that need an older Unity version, this dll should help you.