Simplify usage of user provided aggregation operations.

Add the possibility to provide aggregation operations by leveraging plain java driver API (`Document`/`Bson`).

```java
AggregationOperation op = Aggregation.stage(Aggregates.search(exists(fieldPath("tomatoes.dvd"))));
``` 

When using `Bson` values as part of the stage, we'd need to make sure when converting those to `Document` that the configured `CodecRegistry` is applied correctly.
