Decide when an apphost should be created

[sbomer]

Framework-dependent apphost via /p:UseAppHost=true was implemented in #2282, and #2462 made UseAppHost=true by default. The result is that dotnet publish --self-contained false --rid win-x64, for example, produces a framework-dependent app with an apphost. See some related discussion in dotnet/cli#9460 about exposing this via a --mode option. AFAIK the conclusion was that --self-contained false --rid win-x64 will be the way to get a framework-dependent apphost.

The remaining question is:

• Should we ever generate an apphost when no rid was specified? This was suggested in .NETCore apps cannot specify a Windows application manifest and other things that must go in exe #1899 (comment).

This might reduce to

• Should we set the RID implicitly in some circumstances (wpf/winforms for example)?

[peterhuene]

I'm currently tracking the work to enable this feature in https://github.com/dotnet/cli/issues/10043.

[nguerrera]

I believe this is done. We picked defaults.