Add CAGRA build heuristics independent of HNSW parameters

[achirkin]

Split the cagra::index_params::from_hnsw_params into a core cagra::index_params::from_dataset and a thin wrapper ``cagra::index_params::from_hnsw_params`. This solves two problems:

1. Avoid using the HNSW parameters that do not make sense for CAGRA
2. Allow HNSW parameter heuristics change without affecting CAGRA in future

The new parameter defaults are selected to yield exactly the same configurations at old call sites of from_hnsw_params

[tfeher]

Thanks Artem, it makes sense to separate the dataset shape dependent CAGRA heuristics to general CAGRA heuristic and HNSW specific ones. The PR looks good to me, apart from two small docstring issues.