Allow dev environments to auto-enable dev-centric features in "slim" host builder

[DamianEdwards]

The new "slim" host builder API (#32485) removes a number of default features that exist to make the development time better. It would be great to allow for the development environment (e.g. Visual Studio, dotnet run, dotnet watch, etc.) to be able to re-enable these features when launching the application during the development inner-loop experience so that even when using the slim builder these dev-centric features are still enabled when launched from the developer environment.

The features in question:

• User secrets configuration provider
• Debug logger
• IHostingStartup (enables things like the browser refresh middleware to be injected so the browser can be refreshed on code-change, aka Hot Reload)

This could be achieved by adding a new runtime switch that the slim host builder API would check and if enabled would add these features to the host builder. The switch can be set by the launching environment by passing a build property when building the project, e.g. -p:EnableWebApplicationBuilderDevelopmentFeatures=true (super bad name but you get the idea).

[DamianEdwards]

@eerhardt @davidfowl

[javiercn]

@DamianEdwards It would be great if we can do this from libraries and MSBuild. In other words, compliment this with the ability for libraries to bring in the development features without additional gestures.

There is another case where we use this:

• SPAProxy middleware, which uses hosting startup.

It would also be good to be able to plug-in Blazor Debugging support in hosted projects in a similar way, rather than have it be part of the pipeline.

[ghost]

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
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