traefik plugin 0.7.7 (#73)

* Automatic discovery of the scopes.

Issue #61 raised very valid concerns about users configuring scopes that are not supported by the provider.
This change introduces automatic discovery of supported scopes by fetching the provider's discovery document and filtering out unsupported scopes.

Before:
User configures: scopes: ["openid", "profile", "email", "offline_access"]
Self-hosted GitLab: "The requested scope is invalid, unknown, or malformed"
Authentication: ❌ FAILS

After:
User configures: scopes: ["openid", "profile", "email", "offline_access"]
Middleware checks discovery doc → offline_access not supported
Automatically filters to: ["openid", "profile", "email"]
Authentication: ✅ SUCCEEDS

* Resolves issue #74 by enabling user to specify expected audience in the configuration.

* Fix flaky tests.

audience_test.go

@@ -0,0 +1,143 @@
+package traefikoidc
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"testing"
+)
+
+// TestAudienceConfiguration tests the custom audience configuration feature
+func TestAudienceConfiguration(t *testing.T) {
+	tests := []struct {
+		name             string
+		configAudience   string
+		clientID         string
+		expectedAudience string
+	}{
+		{
+			name:             "no custom audience - uses clientID",
+			configAudience:   "",
+			clientID:         "test-client-id",
+			expectedAudience: "test-client-id",
+		},
+		{
+			name:             "custom audience specified",
+			configAudience:   "api://custom-audience",
+			clientID:         "test-client-id",
+			expectedAudience: "api://custom-audience",
+		},
+		{
+			name:             "auth0 style custom audience",
+			configAudience:   "https://api.example.com",
+			clientID:         "test-client-id",
+			expectedAudience: "https://api.example.com",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Create config with custom audience
+			config := CreateConfig()
+			config.ProviderURL = "https://provider.example.com"
+			config.ClientID = tt.clientID
+			config.ClientSecret = "test-secret"
+			config.SessionEncryptionKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+			config.CallbackURL = "/callback"
+			config.Audience = tt.configAudience
+
+			// Create middleware instance
+			next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.WriteHeader(http.StatusOK)
+			})
+
+			traefikOidc, err := NewWithContext(context.Background(), config, next, "test")
+			if err != nil {
+				t.Fatalf("Failed to create middleware: %v", err)
+			}
+
+			// Verify audience is set correctly
+			if traefikOidc.audience != tt.expectedAudience {
+				t.Errorf("Expected audience %s, got %s", tt.expectedAudience, traefikOidc.audience)
+			}
+
+			// Cleanup
+			traefikOidc.Close()
+		})
+	}
+}
+
+// TestAudienceValidation tests the audience validation in Config.Validate()
+func TestAudienceValidation(t *testing.T) {
+	tests := []struct {
+		name          string
+		audience      string
+		expectError   bool
+		errorContains string
+	}{
+		{
+			name:        "valid custom audience URL",
+			audience:    "https://api.example.com",
+			expectError: false,
+		},
+		{
+			name:        "valid azure style audience",
+			audience:    "api://12345678-1234-1234-1234-123456789012",
+			expectError: false,
+		},
+		{
+			name:        "empty audience is valid (uses clientID)",
+			audience:    "",
+			expectError: false,
+		},
+		{
+			name:          "http URL not allowed",
+			audience:      "http://api.example.com",
+			expectError:   true,
+			errorContains: "audience URL must use HTTPS",
+		},
+		{
+			name:          "wildcard not allowed",
+			audience:      "https://*.example.com",
+			expectError:   true,
+			errorContains: "audience must not contain wildcards",
+		},
+		{
+			name:          "too long audience",
+			audience:      "https://" + string(make([]byte, 250)) + ".com",
+			expectError:   true,
+			errorContains: "audience must not exceed 256 characters",
+		},
+		{
+			name:          "invalid characters",
+			audience:      "api://test\ninjection",
+			expectError:   true,
+			errorContains: "audience contains invalid characters",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			config := CreateConfig()
+			config.ProviderURL = "https://provider.example.com"
+			config.ClientID = "test-client"
+			config.ClientSecret = "test-secret"
+			config.SessionEncryptionKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+			config.CallbackURL = "/callback"
+			config.Audience = tt.audience
+
+			err := config.Validate()
+			if tt.expectError {
+				if err == nil {
+					t.Errorf("Expected error but got none")
+				} else if tt.errorContains != "" && !strings.Contains(err.Error(), tt.errorContains) {
+					t.Errorf("Expected error containing '%s', got: %v", tt.errorContains, err)
+				}
+			} else {
+				if err != nil {
+					t.Errorf("Unexpected error: %v", err)
+				}
+			}
+		})
+	}
+}