[Idea] Implement validation functionality

[bboure]

AppSync/GraphQL already does some heavy lifting about validation. Field types and optionality/requirement are already handled. However, there are some times when you just need more. eg: - a string should be > n characters

• a number has a min/max limit
• a date cannot be in the past/future

etc.

I think it might be a good idea to implement a solution to allow a second layer of validation at the code level for deeper control. Maybe averaging a library like Joi or Yup, with possibly an option for custom implementation.

I see at least 3 things that could use validation.

• event.arguments
• event.source
• the resolver output

The API might look like this:

middy(...).use(appSync({
	validate: {
 	  arguments: ...,
 	  source: ...,
 	  output: ...,
	},
}));

By default, no validation should be made.