Follow up on API design discussion for multi-partition CAGRA search

[jamxia155]

#2035 introduces multi-partition CAGRA search by overloading the search API in the CAGRA public header. A discussion on the PR suggests creating a new multi-partition index class that provides an abstraction for a collection of indices to be searched together:

template <typename BaseIndex>
struct multi_partition_index {
  std::vector<BaseIndex> partitions;
}