Move vector search from IndexInput to RandomAccessInput

### Description

Vector search currently loads vectors from disk by issuing a `seek()` followed by a `readFloats()`. We should instead:
- Add an absolute `readFloats()` method to `RandomAccessInput`
- Refactor the latest vector search file format to use `RandomAccessInput` instead of `IndexInput` to read vectors from disk.

Start by locating RandomAccessInput and the latest vector search file-format implementation, then inspect how vectors are currently read through IndexInput.seek() and readFloats(). Done means RandomAccessInput exposes an absolute readFloats() method and the latest format reads vectors through it, with relevant tests passing.

Written by the indexing model from the issue text.