[FEA] cuvs::neighbors::index to define common methods and arguments

[cjnolet]

Currently, we define a common base class for the knn/ann index but we don't yet define standardized methods / arguments for build, search, and extend. We should define these and make sure the APIs are all consistent across all algos.

The benefit to this is polymorphism, which allows us to start creating higher-level APIs like tiered index and multi-gpu capabilities that only rely on the index, without regard to the type of index.