[jvm-packages] Create demo and test for xgboost4j early stopping.

[trivialfis]

Close #6893

[trivialfis]

@hcho3 Do you know whether the jvm examples are tested on CI?

[codecov-commenter]

Codecov Report

Merging #7252 (e80139f) into master (475fd1a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7252   +/-   ##
=======================================
  Coverage   82.76%   82.76%           
=======================================
  Files          13       13           
  Lines        4061     4061           
=======================================
  Hits         3361     3361           
  Misses        700      700           

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