Add covariance estimators

[ashvardanian]

Covariance isn't a distance function, as it can be negative. It however, is often used for similarity search over time-series and should be implemented in SimSIMD.

[amirzia]

I would like to take this task!

[ashvardanian]

Thanks, @amirzia! I think include/simsimd/probability.h is a good place for those 🤗