Add Microsoft.Extensions.Features to Kestrel slnf

[JamesNK]

Fixes Kestrel.sln failing to build after git clean

[ghost]

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[davidfowl]

@Pilchie do you know if there's a way to automate these things? It's super annoying. I'm sure this needs to be added to basically all the slnf files and nothing builds in VS anymore.

[Pilchie]

Here's the options I can think of:

1. Load each slnf in VS and right click on a project and choose "Load All Dependencies"
2. Parse the project files yourself (or with msbuild, Roslyn's MSBuildWorkspace, etc), find the dependencies and write them.
3. git grep for the project that was split in slnf files, and then add the new project to all of them (assumes we had it mostly right before).

[davidfowl]

@Tratcher can you add these references to all of the slnf files in the repo? I'm guessing everything is broken now without them. We should file an infrastructure issue for doing this.

cc @dougbu

[Tratcher]

I'll take a look

[dougbu]

do you know if there's a way to automate these things?

Note we have a number of *NoDeps.slnf files which intentionally do not include all dependencies. The expectation is probably that users of those solution filters would execute build.cmd in (say) src/Mvc/ before using startvs.cmd there.