Add schema inference support at DataFrame.transform

[HyukjinKwon]

This PR adds the support for schema inference support at DataFrame.transform.

Now, DataFrame.transform can be used as below:

import databricks.koalas as ks

ks.range(10).transform(lambda x: x ** 2)

[HyukjinKwon]

We should add this as a configuration as well later.

[HyukjinKwon]

cc @RainFung FYI

[softagram-bot]

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[codecov-io]

Codecov Report

Merging #732 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
+ Coverage    94.3%   94.33%   +0.02%     
==========================================
  Files          32       32              
  Lines        5569     5580      +11     
==========================================
+ Hits         5252     5264      +12     
+ Misses        317      316       -1

Impacted Files	Coverage Δ
databricks/koalas/frame.py	94.97% <100%> (+0.11%)	⬆️

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[HyukjinKwon]

Merged.