Add data validation to the node server request handler

[MartyO256]

This pull request aims at adding data validation to the request handlers used in the node server for adding entries to the MongoDB.
Since req.body's shape is based on user-controlled input, it should not always be trusted.

I introduce a Joi schema matching the following record type:

learn-ocaml/src/app/learnocaml_exercise_main.ml

Lines 74 to 79 in 82ae88e

	type mongo_solution =
	{ student_id : string;
	timestamp : string;
	collection : string;
	student_solution : string
	}

The following code snippet is responsible for making POST requests from the LearnOCaml server to the node server.
It seems like the mongo_solution is marshalled into an object with the proper keys.

learn-ocaml/src/app/learnocaml_exercise_main.ml

Lines 98 to 102 in 82ae88e

	let student_json = Json.output {student_id = stId; timestamp = current_time; collection = collection_name; student_solution = solution} in
	let nodeRequest = XmlHttpRequest.create () in
	nodeRequest ## _open (Js.string "POST") (url) (Js.bool true);
	nodeRequest ## setRequestHeader (Js.string "Content-Type") (Js.string "application/json; charset=UTF-8");
	nodeRequest ## send (Js.some student_json);

I was surprised to find in the node request handler that the mongo_solution gets unmarshalled as an array.
Is this a limitation of js_of_ocaml?

learn-ocaml/node/app.js

Lines 32 to 43 in 82ae88e

	const split_array = req.body;
	const collection = split_array[3];
	let parsedSolStr = split_array[4];
	const obj = new Object();
	obj.studentId = split_array[1];
	obj.timestamp = new Date().toString();
	obj.solution = parsedSolStr;
	const jsonString = JSON.stringify(obj);
	const solution = JSON.parse( jsonString ); // parse req.body as an object
	db.collection(collection).insertOne(solution);
	console.log(solution);
	res.sendStatus(200); // success status

Testing would be required to ensure that this PR does not break unmarshalling of the request's body.