* adding network file validator contract
* updating to dotnet 5
* reverting change in pipeline
* updating target framework to 5.0
* updating srgen and using target framework var
* reverting srgen back to 3.1
* Adding dotnet 3.1 to build
* Fixing task def
* removing global json
* updating other 3.1 refs to 5.0
* removing netcore folder from build.cake
* Moving and renaming dotnet 3.1 task
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