Move Covariance (Population) covar_pop to be a UDAF #10389
Description
Is your feature request related to a problem or challenge?
Follow-up #10372. Move Covariance Population to datafusion/functions-aggregate/src/covariance.rs
Part of #8708
It should be quite similar to #10372
Describe the solution you'd like
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Describe alternatives you've considered
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Additional context
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