Can potentially avoid HEAD call when opening vector indexes

[westonpace]

In the open_vector_index_v2 method (lance/src/index/vector.rs) the index files are opened. However, we are not using the known sizes that are present in the manifest. This means we then do a HEAD call to get the size when we are reading those files. This could be avoided if we used open_with_size instead of open:

For example:

let aux_path = index_dir.clone().join(uuid).join(INDEX_AUXILIARY_FILE_NAME);
// The below call should be open_with_size!
let aux_reader = object_store.open(&aux_path).await?;

[gstamatakis95]

Don't know if this aligns with the team's direction but I came up with this implementation. Looking for feedback as it could be incomplete.