Allow escaped `:` char in route path

[aldas]

Discussed in #1986

^{Originally posted by morilog September 16, 2021}
Can I able to escape : char in route path to achieve compatiblity with google API design custom methods? something like fiber framework

For example in https://service.name/v1/some/resource/name:customVerb, the :customVerb is not route parameter and only is a simple string.

// curl -v -X POST "http://localhost:8088/v1/some/resource/name:undelete"
func main() {
	e := echo.New()

	// route path as Cloud API "custom method"
	// https://cloud.google.com/apis/design/custom_methods
	e.POST("/v1/some/resource/name\\:undelete", func(c echo.Context) error {
		resp := fmt.Sprintf("mehtod: %v\n", c.Request().Method)
		return c.String(200, resp)
	})
	log.Fatal(e.Start(":8088"))
}