Use `Eigen::BlockSparseMatrix` in Embedded Laplace approximation internals

Eigen 5+ has `BlockSparseMatrix` which we can use for the block diagonal matrix [`W_r`](https://github.com/stan-dev/math/blob/develop/stan/math/mix/functor/laplace_marginal_density_estimator.hpp#L604) in the internals for the Laplace marginal density estimator functions. 
 
https://gitlab.com/libeigen/eigen/-/merge_requests/2644