Support server-side document validation

Starting with version 3.2, MongoDB supports document validation on the server: https://docs.mongodb.com/manual/core/document-validation/

It would be pretty cool if MongoEngine could create and manage the validator, similar to how it already creates and manages indexes. For example, if I have a class like

class Foo:
    name = StringField(required=True)
    x = IntegerField(min_value=1)

MongoEngine could create a validator like

db.runCommand( {
   collMod: "foo",
   validator: { name: {$type: "string"}, x: {$type: "number", $gte: 1} },
   validationLevel: "strict"
} )

This would be useful because:

• if a developer or a script accesses the MongoDB server, bypassing MongoEngine, the server-side validation will still be enforced
• if you use MongoEngine's update method, the server-side validation will still be enforced (this would partially address https://github.com/MongoEngine/mongoengine/issues/1287)

Start by reading MongoEngine's existing index creation and management behavior, then review MongoDB's document validation documentation and the update method context mentioned in the issue. Done means MongoEngine can create and manage validators derived from document fields, with server-side validation enforced for direct access and updates.