✨ v3 (feature): convert fiber.Ctx type to interface

[efectn]

Now we can customize ctx methods as we want. Also much helper methods for ctx were added into the ctx and app.

Inspired by: Echo v5
Closes: #1824

Example:

type customCtx struct {
	fiber.DefaultCtx
}

func (c *customCtx) Params(key string, defaultValue ...string) string {
	return "prefix_" + c.DefaultCtx.Params(key)
}

app.NewCtxFunc(func(app *fiber.App) fiber.CustomCtx {
	return &customCtx{
		DefaultCtx: *fiber.NewDefaultCtx(app),
	}
})

// id = 1
app.Get("/:id", func(c fiber.Ctx) error {
	return c.SendString(c.Params("id")) // example: prefix_1
})