refactor: unify Cookie API to return string and map[string]string

[kk0829]

Summary

This PR unifies the Cookie API to align with the framework's design style.

Breaking Changes

• Cookie() return type changed from *fasthttp.Cookie to string
• Cookies() return type changed from []*fasthttp.Cookie to map[string]string

Bug Fixes

• Fixed dangling pointer bug in internal cookie() method

API Changes

Before

cookie := ctx.Cookie("sid")
if cookie != nil {
    value := string(cookie.Value())
}

for _, c := range ctx.Cookies() {
    fmt.Println(string(c.Key()), string(c.Value()))
}

After

value := ctx.Cookie("sid")
if value != "" {
    // use value
}

for name, value := range ctx.Cookies() {
    fmt.Println(name, value)
}

Benefits

1. No longer exposes underlying fasthttp types
2. Consistent with other framework methods (Query, Param, Header)
3. Simpler usage, no nil checks needed
4. Fixed memory safety bug