fix(binding): Expose validator engine used by the default Validator (#1277)

* fix(binding): Expose validator engine used by the default Validator

- Add func ValidatorEngine for returning the underlying validator engine used
  in the default StructValidator implementation.
- Remove the function RegisterValidation from the StructValidator interface
  which made it immpossible to use a StructValidator implementation without the
  validator.v8 library.
- Update and rename test for registering validation
  Test{RegisterValidation => ValidatorEngine}.
- Update readme and example for registering custom validation.
- Add example for registering struct level validation.
- Add documentation for the following binding funcs/types:
  - Binding interface
  - StructValidator interface
  - Validator instance
  - Binding implementations
  - Default func

* fix(binding): Move validator engine getter inside interface

* docs: rm date cmd from custom validation demo

README.md

@@ -564,7 +564,11 @@ func bookableDate(
 
 func main() {
 	route := gin.Default()
-	binding.Validator.RegisterValidation("bookabledate", bookableDate)
+
+	if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
+		v.RegisterValidation("bookabledate", bookableDate)
+	}
+
 	route.GET("/bookable", getBookable)
 	route.Run(":8085")
 }
@@ -580,13 +584,16 @@ func getBookable(c *gin.Context) {
 ```
 
 ```console
-$ curl "localhost:8085/bookable?check_in=2017-08-16&check_out=2017-08-17"
+$ curl "localhost:8085/bookable?check_in=2018-04-16&check_out=2018-04-17"
 {"message":"Booking dates are valid!"}
 
-$ curl "localhost:8085/bookable?check_in=2017-08-15&check_out=2017-08-16"
+$ curl "localhost:8085/bookable?check_in=2018-03-08&check_out=2018-03-09"
 {"error":"Key: 'Booking.CheckIn' Error:Field validation for 'CheckIn' failed on the 'bookabledate' tag"}
 ```
 
+[Struct level validations](https://github.com/go-playground/validator/releases/tag/v8.7) can also be registed this way.
+See the [struct-lvl-validation example](examples/struct-lvl-validations) to learn more.
+
 ### Only Bind Query String
 
 `ShouldBindQuery` function only binds the query params and not the post data. See the [detail information](https://github.com/gin-gonic/gin/issues/742#issuecomment-315953017).