DATAMONGO-1491 - Add support for $filter (aggregation).

We new support $filter in aggregation pipeline.

Aggregation.newAggregation(Sales.class,
	Aggregation.project()
		.and(filter("items").as("item").by(GTE.of(field("item.price"), 100)))
		.as("items"))

Original pull request: #412.

src/main/asciidoc/reference/mongodb.adoc

@@ -1686,7 +1686,7 @@ At the time of this writing we provide support for the following Aggregation Ope
 | eq (*via: is), gt, gte, lt, lte, ne
 
 | Array Aggregation Operators
-| size, slice
+| size, slice, filter
 
 | Conditional Aggregation Operators
 | cond, ifNull