Add support for instantiating the startup class

[davidfowl]

• Adds an overload of UseStartup that takes a factory so users can control the instance creation. The factory is given the WebHostBuilderContext to expose access to configuration and IWebHostEnvironment.
• Make sure only one startup delegate runs, the last one registered.

Fixes #19809

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
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[Tratcher]

Looks good. Why is it still draft?

[davidfowl]

Looks good. Why is it still draft?

I need to add a test and do API review.

[halter73]

So this adds new public UseStartup method overloads that take a factory to HostingStartupWebHostBuilder and WebHostBuilderExtensions? Anything else? Do we have a good way to create an API diff yet?

[davidfowl]

So this adds new public UseStartup method overloads that take a factory to HostingStartupWebHostBuilder and WebHostBuilderExtensions? Anything else? Do we have a good way to create an API diff yet?

That's the only new API.

[pranavkm]

API looks fine by me. Feel free to send an email for broader visibility.