Build nuget package, support netstandard2.0 build

[HebaruSan]

Motivation

@cheese3660 and @jan-bures have expressed an interest in using some functionality from CKAN in a future update of the KSP2 mod SpaceWarp. Currently this would require them to compile it themselves, which is not very convenient or maintainable.

This dovetails with a longstanding aspiration to make CKAN's core DLL available on Nuget for mods.

Changes

• Now the core DLL has a third target framework, netstandard2.0, because it is better for mods
• Now the core DLL has properties to autogenerate a .nupkg file containing DLLs for net48, net7.0, and netstandard2.0
• Now the Cake build script is updated to perform a separate all-platforms build for the core DLL on Linux, which is required for the .nupkg file to be created (Windows already handles it in the single build-everything pass that we're already doing)
• Now once we define NUGET_API_KEY in this repo's secrets, it will be used to upload the generated .nupkg file to Nuget when we make a release

Fixes #1031.
Fixes #1037.
Fixes #1588.

Important note

We have no established commitment to follow SemVer with respect to the core DLL's public types. The only current versioning rule is that we increment the minor version when we change the metadata spec/schema in ways that older clients can't support, and the patch version otherwise (which is always even for stable releases and odd for dev builds).

We might choose to make such a commitment in the future, but be aware that for now, if you use this DLL, any public stuff might change from one version to the next, so you should only update its dependency version if you are prepared to fix an unknown number of compile errors.