Add prefercliruntime file to dotnet-codegen CLI tool

[amis92]

It might or might not resolve issues like #84 (no 1.0 shared fx).

Pending check.

The file usage is not yet (well) documented, pending https://github.com/dotnet/docs/issues/7292

[AArnott]

I expect #86 will also take care of #84. I'll check it today. If so, I may not take this PR since it uses an as-yet undocumented feature. But thank you for the submission either way.

[AArnott]

I've verified that #86 fixes #84. Closing this.

[amis92]

• The feature is used widely across a lot of official tools
• It's a part of test suites in dotnet CLI (https://github.com/dotnet/cli/commit/dc876803377abd715f53b5f7eeb7ac750113222d)
• It's visibly part of master (and works since at least v2.0):
  • current: https://github.com/dotnet/cli/blob/c8a8d2caabc9946837e5fd9439ab99000ab19d27/src/dotnet/CommandFactory/CommandResolution/PackagedCommandSpecFactoryWithCliRuntime.cs
  • previous: https://github.com/dotnet/cli/blob/7e8de1bd32ef14d257235e44a75bf391e230642d/src/Microsoft.DotNet.Cli.Utils/CommandResolution/PackagedCommandSpecFactoryWithCliRuntime.cs
  • original introduction commit: https://github.com/dotnet/cli/commit/dc876803377abd715f53b5f7eeb7ac750113222d

Benefits provided, according to an implementation:

• if the dotnet CLI running the build is using a sharedfx with the same major version as the one required by the dotnet-codegen, it'll override and run the tool under that framework
• if the major versions differ, the operation fails

Currently, we require customers using the generators to run the build on a machine with .NET Core v2.0 runtime available, which is currently out of support officially. It's not critical, but it's a kind of a papercuts issue.

Also no issue with compatibility since we target the lowest applicable, netcoreapp2.0 (it'll run on every .NET Core runtime v2.x)

[amis92]

Sooo now there's a bit more "official" documentation on the topic of prefercliruntime: https://github.com/dotnet/docs/issues/7292#issuecomment-497031254

Comment added for future references