Add Spark 3.0 build.

[ueshin]

This is a temporary build for Spark 3.0-RC. We should refine after Spark 3.0 is released.

[itholic]

Good preparation! when will the Spark 3.0.0 be officially released ?

[codecov-commenter]

Codecov Report

Merging #1558 into master will increase coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1558      +/-   ##
==========================================
+ Coverage   94.15%   94.46%   +0.31%     
==========================================
  Files          38       38              
  Lines        8600     8696      +96     
==========================================
+ Hits         8097     8215     +118     
+ Misses        503      481      -22     

Impacted Files	Coverage Δ
databricks/koalas/base.py	97.36% <ø> (+0.09%)	⬆️
databricks/koalas/config.py	99.00% <ø> (+0.01%)	⬆️
databricks/koalas/frame.py	96.64% <ø> (+0.88%)	⬆️
databricks/koalas/generic.py	96.66% <ø> (+0.06%)	⬆️
databricks/koalas/groupby.py	90.41% <ø> (+0.09%)	⬆️
databricks/koalas/indexes.py	96.83% <ø> (+0.03%)	⬆️
databricks/koalas/internal.py	97.47% <ø> (+0.75%)	⬆️
databricks/koalas/ml.py	96.29% <ø> (ø)
databricks/koalas/namespace.py	86.09% <ø> (ø)
databricks/koalas/plot.py	94.36% <ø> (+0.08%)	⬆️
... and 23 more

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[HyukjinKwon]

Good preparation! when will the Spark 3.0.0 be officially released ?

Hopefully this month

[HyukjinKwon]

LGTM to check Spark 3.0 compat. We might have to revert and/or update later in order to use the official Spark 3.0 though :-)

[ueshin]

Thanks! merging.
I'm fine with reverting and/or updating later. Let's do that when Spark 3.0 is released.