Multi-platform build process (#1928)

* Add x86 to build process.

- Use recommended structure for multi-platform nuget.
- Separate build and pack to allow x64 and x86 builds.
- Use matrix in Github Actions to reduce repetition.

* Properly implement multi-platform nuget build.

- Split build pipeline to setting version, building and packing.
- Build AnyCPU only as reference assemblies.
- Add ref folder to nuget.

* Restructure project and pipeline.

Based on PR review discussion.

- Restore default x64,x86 build process.
  - Build generates platform specific nupkg.
  - Remove AnyCPU platform to prevent confusion.
- Add custom property for building reference libraries.
  - AnyCPU is only set by the property.
- Move nuget changes to separate target file.
  - Imported only when custom property is set.
  - Clear separation from the default build.
- Merge frameworks in published library artifacts.
- Re-simplify pipelines; no matrix or separate jobs.