Enable External Durable Client Managed Identity Support (#2856)

* Update StandardConnectionInfoProvider.cs

* Update StandardConnectionInfoProvider.cs

* Update StandardConnectionInfoProvider.cs

* Update StandardConnectionInfoProvider.cs

* add description about the resolve method implementation

* remove whitespace

* update by comment

src/WebJobs.Extensions.DurableTask/StandardConnectionInfoProvider.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
 using System;
+using System.Linq;
 using Microsoft.Extensions.Configuration;
 
 namespace Microsoft.Azure.WebJobs.Extensions.DurableTask
@@ -23,10 +24,40 @@ public StandardConnectionInfoProvider(IConfiguration configuration)
             this.configuration = configuration ?? throw new ArgumentNullException(nameof(configuration));
         }
 
+        // This implementation is a clone of `IConfigurationSection.Exists` found here https://github.com/dotnet/runtime/blob/main/src/libraries/Microsoft.Extensions.Configuration.Abstractions/src/ConfigurationExtensions.cs#L78
+        // Functions host v1 (.net462 framework) doesn't support this method so we implement a substitute one here.
+        private bool IfExists(IConfigurationSection section)
+        {
+            if (section == null)
+            {
+                return false;
+            }
+
+            if (section.Value == null)
+            {
+                return section.GetChildren().Any();
+            }
+
+            return true;
+        }
+
         /// <inheritdoc />
         public IConfigurationSection Resolve(string name)
         {
-            return this.configuration.GetSection(name);
+            // This implementation is a replica of the WebJobsConnectionInfoProvider used for the internal durable client.
+            // The original code can be found at:
+            // https://github.com/Azure/azure-functions-durable-extension/blob/dev/src/WebJobs.Extensions.DurableTask/WebJobsConnectionInfoProvider.cs#L37.
+            // We need to first check the configuration section with the AzureWebJobs prefix, as this is the default name within the Functions app whether it's internal or external.
+            string prefixedConnectionStringName = "AzureWebJobs" + name;
+            IConfigurationSection section = this.configuration?.GetSection(prefixedConnectionStringName);
+
+            if (!this.IfExists(section))
+            {
+                // If the section doesn't exist, then look for the configuration section without the prefix, since there is no prefix outside the WebJobs app.
+                section = this.configuration?.GetSection(name);
+            }
+
+            return section;
         }
     }
-}
+}