add KV extension method for service principal

src/Impl/Azure/Config.Net.Azure.KeyVault/AzureKeyVaultConfigStore.cs

@@ -1,7 +1,10 @@
 using System;
+using System.Net.Http;
+using System.Threading.Tasks;
 using Microsoft.Azure.KeyVault;
 using Microsoft.Azure.KeyVault.Models;
 using Microsoft.Azure.Services.AppAuthentication;
+using Microsoft.IdentityModel.Clients.ActiveDirectory;
 
 namespace Config.Net.Azure.KeyVault
 {
@@ -25,6 +28,31 @@ public static AzureKeyVaultConfigStore CreateWithManagedIdentity(Uri vaultUri)
          return new AzureKeyVaultConfigStore(vaultUri, client);
       }
 
+      public static AzureKeyVaultConfigStore CreateWithPrincipal(Uri vaultUri, string azureAadClientId, string azureAadClientSecret)
+      {
+         var credential = new ClientCredential(azureAadClientId, azureAadClientSecret);
+
+         var client = new KeyVaultClient(
+            new KeyVaultClient.AuthenticationCallback( (authority, resource, scope) => GetAccessToken(authority, resource, scope, credential) ),
+            GetHttpClient());
+
+         return new AzureKeyVaultConfigStore(vaultUri, client);
+      }
+
+      private static async Task<string> GetAccessToken(string authority, string resource, string scope, ClientCredential credential)
+      {
+         var context = new AuthenticationContext(authority, TokenCache.DefaultShared);
+
+         AuthenticationResult result = await context.AcquireTokenAsync(resource, credential);
+
+         return result.AccessToken;
+      }
+
+      private static HttpClient GetHttpClient()
+      {
+         return new HttpClient();
+      }
+
       public bool CanRead => true;
 
       public bool CanWrite => false;

src/Impl/Azure/Config.Net.Azure.KeyVault/ConfigurationExtensions.cs

@@ -12,5 +12,14 @@ public static ConfigurationBuilder<TInterface> UseAzureKeyVaultWithManagedIdenti
 
          return builder;
       }
+
+      public static ConfigurationBuilder<TInterface> UseAzureKeyVaultWithServicePrincipal<TInterface>(
+         this ConfigurationBuilder<TInterface> builder, Uri vaultUri, string clientId, string clientSecret) where TInterface : class
+      {
+         builder.UseConfigStore(AzureKeyVaultConfigStore.CreateWithPrincipal(vaultUri, clientId, clientSecret));
+
+         return builder;
+      }
+
    }
 }