* implement transport binding factory
* split https transport into seperate nuget package
* new Opc.Ua.Binding.Https assembly
* Https binding is loaded on demand, if available
* reduce core dependencies to kestrel and others
fixes#600
- solutions support still VS2017 and VS 2019
- ci now also tests net462 & netcoreapp3.1
- next rev of nuget packages will have netstandard2.1 support
- a few small fixes for netcoreapp3.1