* 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
- use nerdbank.gitversioning to determine devops build version
- split nuget packages into new structure, one package per assembly to allow granular use of packages.
- also build .Debug versions
- back compat at least for a while, the old Opc.Ua packages reference also the new packages.
- devops produces preview builds for master
- preview builds are now signed, including nuget packages
- add license files for nuget packages, since licenseUrl is deprecated