Allow suppressing the use of environment variables

[Tratcher]

#20328 The GenericWebHost and WebHostBuilder automatically add a config provider for "ASPNETCORE_" environment variables with no opportunity to disable it. This is problematic for:

• Test scenarios
• Running two hosts in one process
• Running in a child process where environment variables are inherited by default

This change adds an option to suppress environment based config.

public static IHostBuilder ConfigureWebHost(this IHostBuilder builder, Action<IWebHostBuilder> configure, Action<WebHostBuilderOptions> configureWebHostBuilder);

public WebHostBuilder(WebHostBuilderOptions options);

    public class WebHostBuilderOptions
    {
        /// <summary>
        /// Indicates if "ASPNETCORE_" prefixed environment variables should be added to configuration.
        /// The default is true.
        /// </summary>
        public bool UseEnvironmentConfiguration { get; set; } = true;
    }

// We can leave WebHostBuilder alone
- public WebHostBuilder(WebHostBuilderOptions options);

-  public bool UseEnvironmentConfiguration { get; set; } = true;
+  public bool SuppressUseEnvironmentConfiguration { get; set; }

[davidfowl]

Why is this on the HostBuilder?

[Tratcher]

Why is this on the HostBuilder?

Because it has to be called before ConfigureWebHost/Defaults in order to affect their behavior.

[davidfowl]

This needs an API review. I'm not a fan of having this be an extension method on the IHostBuilder that affects the webshost builder scenario. We might need another way to do this (like passing a bool to ConfigureWebHost).

[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.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[ghost]

Hello @Tratcher!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.