How can you create custom directives in GraphQL, and what are some use cases for them?

Custom directives in GraphQL can be defined in the schema language and implemented on the server side, typically in the GraphQL server configuration. These directives can be used to change how queries or mutations are executed; for example, they can be used to perform field-level transformations, enforce permissions, or implement custom business logic. Some use cases include logging, authentication, field deprecation, and dynamically changing query responses based on certain conditions or user roles.