Allow minimal host to be created without default HostBuilder behavior

[eerhardt]

This adds a new Hosting API to reduce startup and app size, and ensures the default behavior is NativeAOT compatible.

Fix #32485

Notes:

1. I wasn't sure how to refactor SlimWebHostBuilder and GenericWebHostBuilder to share more code. If you have thoughts or ideas, please let me know. Should they inherit from a common base class?
2. Similar question for the new WebApplicationBuilder constructor.

[eerhardt]

I think this PR is ready to be reviewed, if anyone wants to take a look.

[halter73]

We have not already applied "ASPNETCORE_" environment variables to hosting config in this code path. Only "DOTNET_". We should do both. I bet this is why the macOS template test is failing to load the server certificate.

We had an issue a while back with the template tests using expired dev certs on macOS. That was fixed by using "ASPNETCORE_"-prefixed environment variables to set a default cert, so the tests shouldn't rely on the development cert being up to date.

aspnetcore/src/ProjectTemplates/Shared/AspNetProcess.cs

Lines 91 to 95 in 40dd230

	var finalEnvironmentVariables = new Dictionary<string, string>(environmentVariables)
	{
	["ASPNETCORE_Kestrel__Certificates__Default__Path"] = _developmentCertificate.CertificatePath,
	["ASPNETCORE_Kestrel__Certificates__Default__Password"] = _developmentCertificate.CertificatePassword,
	};

I'm guessing the dev cert is working as a fallback on the other OS's, but we still have that original issue on macOS when it falls back to the dev cert. @adityamandaleeka I wonder if this is related to our wider macOS dev cert issues.

[halter73]

Have you tested how it interacts with WebApplicationFactory? My assumption is that it will get the IHostBuilder from HostApplicationBuilder.HostBuilderAdapter and then add back the GenericWebHostBuilder rather than the SlimWebHostBuilder here.

aspnetcore/src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs

Lines 206 to 211 in c77deeb

	hostBuilder.ConfigureWebHost(webHostBuilder =>
	{
	SetContentRoot(webHostBuilder);
	_configuration(webHostBuilder);
	webHostBuilder.UseTestServer();
	});

Not a big deal in terms of this PR, but something to follow up on. I was talking to @davidfowl about creating a more "minimal" version of WebApplicationFactory, so that could tie in.

I'm also interested in seeing how much emptier we could possibly make this. I know there's a lot more trimming to do in lower layers even for the "slim" mode.

[halter73]

Nit: How easy is it to determine which builder failed if only one does with this member data? It might be nicer to use strings for the MemberData and have it index into a static Dictionary<string, CreateBuilderFunc>. We are often bad at using serializable member data. But it might be worth it here given how often this is used.

[eerhardt]

This actually works pretty well, from what I can tell. In Test Explorer:

    2)   Microsoft.AspNetCore.Tests.WebApplicationTests.AddingMemoryStreamBackedConfigurationWorks(createBuilder: CreateBuilderFunc { Method = Microsoft.AspNetCore.Builder.WebApplicationBuilder CreateSlimBuilder(), Target = null }) 
      Duration: 1 ms

    1)   Microsoft.AspNetCore.Tests.WebApplicationTests.AddingMemoryStreamBackedConfigurationWorks(createBuilder: CreateBuilderFunc { Method = Microsoft.AspNetCore.Builder.WebApplicationBuilder CreateBuilder(), Target = null }) 
      Duration: 3 ms

And when the test fails:

[eerhardt]

Have you tested how it interacts with WebApplicationFactory?

No, I haven't. I don't really know much about it and can try it next week. Or I can log a follow up issue to track ensuring it works as we want it to.

I'm also interested in seeing how much emptier we could possibly make this. I know there's a lot more trimming to do in lower layers even for the "slim" mode.

Yeah, there is much more work to make it smaller. This is just the start to get the API in. Now other work items (one example: #46142) can build on this to trim more out.

My next plan is to get a new API into Extensions.Hosting for an "empty" HostApplicationBuilder. This code can then use that, and trim even more stuff out by default.