Add $rerank aggregation stage support

Adds builder support for the $rerank pipeline stage (MongoDB 8.3, Atlas only).

API:
- RerankQuery: represents the query object (text field, extensible for future modalities)
- RerankOptions: immutable options builder for forward-compatible extensibility
- Aggregates.rerank(): 4 overloads (single/multi path, with/without options)
- Scala wrappers in Aggregates.scala

JAVA-6052

Author: rozza