GraphQL Object constraints (#5715)

* GraphQL Object constraints

Implements the GraphQL Object constraints, which allows us to filter queries results using the `$eq`, `$lt`, `$gt`, `$in`, and other Parse supported constraints.
Example:
```
query objects {
  findMyClass(where: {
    objField: {
      _eq: {
        key: 'foo.bar',
        value: 'hello'
      },
      _gt: {
        key: 'foo.number',
        value: 10
      },
      _lt: {
        key: 'anotherNumber',
        value: 5
      }
    }
  }) {
    results {
      objectId
    }
  }
}
```
In the example above, we have the `findMyClass` query (automatically generated for the `MyClass` class), and a field named `objField` whose type is Object. The object below represents a valid `objField` value and would satisfy all constraints:
```
{
  "foo": {
    "bar": "hello",
    "number": 11
  },
  "anotherNumber": 4
}
```
The Object constraint is applied only when using Parse class object type queries. When using "generic" queries such as `get` and `find`, this type of constraint is not available.

* Objects constraints not working on Postgres

Fixes the $eq, $ne, $gt, and $lt constraints when applied on an Object type field.

* Fix object constraint field name

* Fix Postgres constraints indexes

* fix: Object type composed constraints not working

* fix: Rename key and value fields

* refactor: Object constraints for generic queries

* fix: Object constraints not working on Postgres

spec/Schema.spec.js

@@ -451,7 +451,9 @@ describe('SchemaController', () => {
           )
         )
         .then(actualSchema => {
-          expect(dd(actualSchema.classLevelPermissions, newLevelPermissions)).toEqual(undefined);
+          expect(
+            dd(actualSchema.classLevelPermissions, newLevelPermissions)
+          ).toEqual(undefined);
           done();
         })
         .catch(error => {