[FEA] Standardize `cuVS::neighbors::index`

Currently, all of the Ann indexes inherit from a common base class, but the base class doesn't have any virtual methods or state attached to it, so any APIs coded to the index class can't even assume common functions like build and search. This is making it hard to code new APIs like tiered index and replication / sharding APIs that are agnostic to the index type.

We have been working on generalizing the various index implementations so that we can enable this functionality. This issue is primarily concerned with taking it across the finish line and having all of the Ann index implementations inherit at least their build(), search() and extend() capabilities from the common base (or mixins if more appropriate given the differences).

Start by reviewing the common base class and the various ANN index implementations. Compare how build(), search(), and extend() are currently exposed across the implementations, then determine how the common base or mixins can provide those capabilities. Done means all ANN indexes share these capabilities so APIs such as tiered indexing and replication or sharding can remain index-agnostic.