Reconsider names for ANN related format and APIs

There is some discussion about the codec name for ann search.
https://lists.apache.org/thread.html/r3a6fa29810a1e85779de72562169e72d927d5a5dd2f9ea97705b8b2e%40%3Cdev.lucene.apache.org%3E

Main points here are 1) use plural form for consistency, and 2) use more specific name for ann search (second point could be optional).

A few alternatives were proposed:

• VectorsFormat
• VectorValuesFormat
• NeighborsFormat
• DenseVectorsFormat