Add mean keyword to dpnp.std and dpnp.var

[antonwolfy]

The PR proposes to add mean keyword argument to dpnp.std, dpnp.var, dpnp.nanstd, dpnp.nanvar functions and dpnp.ndarray.std, dpnp.ndarray.var methods.

The keyword was introduced by NumPy 2.0 and intended to improve the performance:

Often when the standard deviation is needed the mean is also needed; the same holds for the variance and the mean. With the current code the mean is then calculated twice, this can be prevented if the functions calculating the variance or the standard deviation can use a precalculated mean

This PR implements similar improvement in dpnp code.

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[coveralls]

coverage: 71.297% (+0.02%) from 71.275%
when pulling e86a139 on add-mean-keyword-to-std-var
into d509d3f on master.

[github-actions]

Array API standard conformance tests for dpnp=0.17.0dev4=py312he4f9c94_19 ran successfully.
Passed: 954
Failed: 0
Skipped: 46

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2271/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.17.0dev5=py312he4f9c94_19 ran successfully.
Passed: 957
Failed: 0
Skipped: 43

[vtavana]

Thank you, @antonwolfy!