[SPARK-40386][PS][SQL] Implement ddof in DataFrame.cov

[zhengruifeng]

What changes were proposed in this pull request?

1, add a dedicated expression for DataFrame.cov;
2, add missing parameter ddof in DataFrame.cov

Why are the changes needed?

for api coverage

Does this PR introduce any user-facing change?

yes, API change

 >>> np.random.seed(42)
>>> df = ps.DataFrame(np.random.randn(1000, 5),
... columns=['a', 'b', 'c', 'd', 'e'])
>>> df.cov()
a b c d e
a 0.998438 -0.020161 0.059277 -0.008943 0.014144
b -0.020161 1.059352 -0.008543 -0.024738 0.009826
c 0.059277 -0.008543 1.010670 -0.001486 -0.000271
d -0.008943 -0.024738 -0.001486 0.921297 -0.013692
e 0.014144 0.009826 -0.000271 -0.013692 0.977795
>>> df.cov(ddof=2)
a b c d e
a 0.999439 -0.020181 0.059336 -0.008952 0.014159
b -0.020181 1.060413 -0.008551 -0.024762 0.009836
c 0.059336 -0.008551 1.011683 -0.001487 -0.000271
d -0.008952 -0.024762 -0.001487 0.922220 -0.013705
e 0.014159 0.009836 -0.000271 -0.013705 0.978775
>>> df.cov(ddof=-1)
a b c d e
a 0.996444 -0.020121 0.059158 -0.008926 0.014116
b -0.020121 1.057235 -0.008526 -0.024688 0.009807
c 0.059158 -0.008526 1.008650 -0.001483 -0.000270
d -0.008926 -0.024688 -0.001483 0.919456 -0.013664
e 0.014116 0.009807 -0.000270 -0.013664 0.975842

How was this patch tested?

added tests

[zhengruifeng]

cc @itholic@HyukjinKwon@xinrong-meng@Yikun

[zhengruifeng]

Merged into master, thank you @HyukjinKwon