Type annotations to Koalas accessors and Spark accessors

[xinrong-meng]

No description provided.

[ueshin]

LGTM, pending tests.

[codecov-io]

Codecov Report

Merging #1902 (5203dc4) into master (674cdd2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1902      +/-   ##
==========================================
+ Coverage   94.14%   94.16%   +0.01%     
==========================================
  Files          41       41              
  Lines        9979     9975       -4     
==========================================
- Hits         9395     9393       -2     
+ Misses        584      582       -2     

Impacted Files	Coverage Δ
databricks/koalas/accessors.py	93.00% <100.00%> (ø)
databricks/koalas/spark/accessors.py	94.96% <100.00%> (ø)
databricks/koalas/utils.py	95.71% <0.00%> (-0.36%)	⬇️
databricks/koalas/indexes.py	96.81% <0.00%> (+0.37%)	⬆️

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[xinrong-meng]

@ueshin Thank you!