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

examples/custom-validation/server.go

@@ -30,7 +30,11 @@ 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")
 }