Expose configuration from worker processes

I am trying to connect my function app to keyvault and get queue name and connection secrets. This was working well with .netcore3.1 app using the ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder) method in FunctionStartup.

After upgrading to .net5 dotnet-isolated, the bindings does not work. I configured azurekeyvault in Program.cs but still it does not pick from keyvault. Seems like functionstartup is trying to bind before Main() is invoked.

**QueueFunction**

public static void Run([QueueTrigger("%QueueName%", Connection = "QueueConnection")] string message, string id)

**Startup.cs (.netcore3.1)- working**

public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
{
var azureKeyVaultURL = Environment.GetEnvironmentVariable("AzureKeyVaultURL");
var azureKeyVaultADAppID = Environment.GetEnvironmentVariable("AzureKeyVaultMIAppID");


builder.ConfigurationBuilder
.SetBasePath(Environment.CurrentDirectory)
.AddAzureKeyVault(new Uri(azureKeyVaultURL), new ManagedIdentityCredential(azureKeyVaultADAppID))
.AddEnvironmentVariables()
.Build();
}

**Program.cs (.net5)- Not working**

var host = new HostBuilder()
.ConfigureFunctionsWorkerDefaults()
.ConfigureAppConfiguration(config =>{
var azureKeyVaultURL = Environment.GetEnvironmentVariable("AzureKeyVaultURL");
var azureKeyVaultADAppID = Environment.GetEnvironmentVariable("AzureKeyVaultMIAppID");

config
.SetBasePath(Environment.CurrentDirectory)
.AddAzureKeyVault(new Uri(azureKeyVaultURL), new ManagedIdentityCredential(azureKeyVaultADAppID))
.AddEnvironmentVariables()
.Build();
})
.Build()

Compare the .NET Core Startup.cs configuration path with Program.cs in the .NET 5 isolated worker, focusing on ConfigureFunctionsWorkerDefaults and ConfigureAppConfiguration. Reproduce the QueueFunction binding with QueueName and QueueConnection values from Azure Key Vault; done means those bindings resolve before invocation in the isolated worker.

Written by the indexing model from the issue text.