azureblob,azurefiles: add support for client assertion based authentication

backend/azurefiles/azurefiles.go

@@ -569,6 +569,38 @@ func newFsFromOptions(ctx context.Context, name, root string, opt *Options) (fs.
 		if err != nil {
 			return nil, fmt.Errorf("failed to acquire MSI token: %w", err)
 		}
+	case opt.ClientID != "" && opt.Tenant != "" && opt.MSIClientID != "":
+		// Workload Identity based authentication
+		var options azidentity.ManagedIdentityCredentialOptions
+		options.ID = azidentity.ClientID(opt.MSIClientID)
+
+		msiCred, err := azidentity.NewManagedIdentityCredential(&options)
+		if err != nil {
+			return nil, fmt.Errorf("failed to acquire MSI token: %w", err)
+		}
+
+		getClientAssertions := func(context.Context) (string, error) {
+			token, err := msiCred.GetToken(context.Background(), policy.TokenRequestOptions{
+				Scopes: []string{"api://AzureADTokenExchange"},
+			})
+
+			if err != nil {
+				return "", fmt.Errorf("failed to acquire MSI token: %w", err)
+			}
+
+			return token.Token, nil
+		}
+
+		assertOpts := &azidentity.ClientAssertionCredentialOptions{}
+		cred, err = azidentity.NewClientAssertionCredential(
+			opt.Tenant,
+			opt.ClientID,
+			getClientAssertions,
+			assertOpts)
+
+		if err != nil {
+			return nil, fmt.Errorf("failed to acquire client assertion token: %w", err)
+		}
 	default:
 		return nil, errors.New("no authentication method configured")
 	}