feat(auth): Implement GitHub OAuth authentication flow for package publishing

- Added authentication service and GitHub OAuth integration.
- Introduced new authentication methods and structures in the model.
- Updated the API to handle authentication during the publishing process.
- Created handlers for starting and checking the status of the OAuth flow.
- Enhanced the publish handler to validate authentication credentials.
- Updated configuration to include GitHub OAuth settings.
- Added tests for the OAuth flow and publishing with authentication.
- Updated documentation to reflect changes in the API and authentication process.

internal/auth/auth.go

@@ -0,0 +1,28 @@
+// Package auth provides authentication mechanisms for the MCP registry
+package auth
+
+import (
+	"context"
+	"errors"
+
+	"github.com/modelcontextprotocol/registry/internal/model"
+)
+
+var (
+	// ErrAuthRequired is returned when authentication is required but not provided
+	ErrAuthRequired = errors.New("authentication required")
+	// ErrUnsupportedAuthMethod is returned when an unsupported auth method is used
+	ErrUnsupportedAuthMethod = errors.New("unsupported authentication method")
+)
+
+// Service defines the authentication service interface
+type Service interface {
+	// StartAuthFlow initiates an authentication flow and returns the flow information
+	StartAuthFlow(ctx context.Context, method model.AuthMethod, repoRef string) (map[string]string, string, error)
+
+	// CheckAuthStatus checks the status of an authentication flow using a status token
+	CheckAuthStatus(ctx context.Context, statusToken string) (string, error)
+
+	// ValidateAuth validates the authentication credentials
+	ValidateAuth(ctx context.Context, auth model.Authentication) (bool, error)
+}