Add support for the AzureSDK (#542)

See https://github.com/tamram/storage-dotnet-azure-ad-msal/tree/tamram-0818

```
[AuthorizeForScopes(Scopes = new string[] { "https://storage.azure.com/user_impersonation" })]
        public async Task<IActionResult> Blob()
        {
            var scopes = new string[] { "https://storage.azure.com/user_impersonation" }; // I guess the Blob SDK knows already?
            ViewData["Message"] = await CreateBlob(new TokenAcquisitionTokenCredential(_tokenAcquisition),);
            return View();
        }

        private static async Task<string> CreateBlob(TokenAcquisitionTokenCredential tokenCredential)
        {
            // Replace the URL below with the URL to your blob.
            Uri blobUri = new Uri("https://storagesamples.blob.core.windows.net/sample-container/blob1.txt");
            BlobClient blobClient = new BlobClient(blobUri, tokenCredential);

            // Create a blob on behalf of the user.
            string blobContents = "Blob created by Azure AD authenticated user.";
            byte[] byteArray = Encoding.ASCII.GetBytes(blobContents);

            using (MemoryStream stream = new MemoryStream(byteArray))
            {
                await blobClient.UploadAsync(stream);
            }
            return "Blob successfully created";
        }
```

src/Microsoft.Identity.Web/AzureSdkSupport/TokenAcquisitionTokenCredential.cs

@@ -0,0 +1,43 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System.Threading;
+using System.Threading.Tasks;
+using Azure.Core;
+using Microsoft.Identity.Client;
+
+namespace Microsoft.Identity.Web
+{
+    /// <summary>
+    /// Azure SDK token credential based on the ITokenAcquisition service.
+    /// </summary>
+    public class TokenAcquisitionTokenCredential : TokenCredential
+    {
+        private ITokenAcquisition _tokenAcquisition;
+
+        /// <summary>
+        /// Constructor from an ITokenAcquisition service.
+        /// </summary>
+        /// <param name="tokenAcquisition">Token acquisition.</param>
+        public TokenAcquisitionTokenCredential(ITokenAcquisition tokenAcquisition)
+        {
+            _tokenAcquisition = tokenAcquisition;
+        }
+
+        /// <inheritdoc/>
+        public override AccessToken GetToken(TokenRequestContext requestContext, CancellationToken cancellationToken)
+        {
+            AuthenticationResult result = _tokenAcquisition.GetAuthenticationResultForUserAsync(requestContext.Scopes)
+                .GetAwaiter()
+                .GetResult();
+            return new AccessToken(result.AccessToken, result.ExpiresOn);
+        }
+
+        /// <inheritdoc/>
+        public override async ValueTask<AccessToken> GetTokenAsync(TokenRequestContext requestContext, CancellationToken cancellationToken)
+        {
+            AuthenticationResult result = await _tokenAcquisition.GetAuthenticationResultForUserAsync(requestContext.Scopes).ConfigureAwait(false);
+            return new AccessToken(result.AccessToken, result.ExpiresOn);
+        }
+    }
+}