[release/6.0] Load IConfigurationProviders once in WebApplicationBuilder

[github-actions]

Backport of #37039 to release/6.0

/cc @halter73

Customer Impact

This PR addresses two major issues:

1. Given config sources that can only be loaded once (such as those backed by Stream), WebApplicationBuilder.Build() throws an InvalidOperationException.

   var builder = WebApplication.CreateBuilder();
   
   using Stream jsonStream = await GetJsonConfig();
   builder.Configuration.AddJsonStream(jsonStream);
   
   using var app = builder.Build();
   
   // ...

   Unhandled exception. System.InvalidOperationException: StreamConfigurationProviders cannot be loaded more than once.
      at Microsoft.Extensions.Configuration.StreamConfigurationProvider.Load()
      at Microsoft.Extensions.Configuration.ConfigurationRoot..ctor(IList`1 providers)
      at Microsoft.Extensions.Configuration.ConfigurationBuilder.Build()
      at Microsoft.Extensions.Hosting.HostBuilder.BuildAppConfiguration()
      at Microsoft.Extensions.Hosting.HostBuilder.Build()
      at Microsoft.AspNetCore.Builder.WebApplicationBuilder.Build()
      at Program.<Main>$(String[] args) in C:\dev\halter73\NewTemplate\Program.cs:line 12
   

2. Changes to configuration sources after build are not observed. This means that if appsettings.json is modified from:

   {
     "Logging": {
       "LogLevel": {
         "Default": "Warning"
       }
     }
   }

   to:

   {
     "Logging": {
       "LogLevel": {
         "Default": "Debug"
       }
     }
   }

   while the application is running, only logs with severity warning and higher will be logged until the application is restarted.

   After this change, a file change notification correctly updates the default log level to Debug and debug-level logs are emitted immediately.

Regression

The inability to notice configuration source changes was a regression between rc1 and rc2.

Testing

Manual testing. Added both end-to-end and unit regression tests.

Risk

Low. This change has undergone significant testing and review.