🚀 v3 Request: fiber.Ctx type of interface

If fiber.Ctx will be interface , we can customize it.

Example

```go
type CustomCtx struct {
	fiber.Ctx
}

func (c *CustomCtx) Foo() {
	println("foo")
}

func (c *CustomCtx) Bar() {
	println("bar")
}

app.Get("/", func(c CustomCtx) error {
	c.Foo()
	c.Bar()
	return c.JSON("")
})
```