feat: add Gin timeout and admin middleware example with tests

[appleboy]

• Add a new example demonstrating Gin timeout and admin middleware usage
• Implement a simple Gin server with timeout and custom auth middleware
• Add a README explaining how to run and test the example, including expected timeout and unauthorized responses
• Add go.mod and go.sum files for the new example with necessary dependencies and a local replace directive
• Fix the timeout middleware to use the original context instead of a copy
• Refactor and expand timeout middleware tests, including a test for using the middleware with gin.Use
• Remove the WithHandler option and associated handler logic from the timeout middleware
• Refactor middleware to always call c.Next() instead of a configurable handler
• Update all usages and tests to remove WithHandler, passing handlers directly instead
• Change test expectations for requests without a timeout to return HTTP 408 Request Timeout and its default message
• Simplify the middleware configuration and usage throughout the codebase

fix #32