Allow kwargs in dask predict

[RAMitchell]

No description provided.

[codecov-commenter]

Codecov Report

Merging #6117 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6117   +/-   ##
=======================================
  Coverage   78.52%   78.52%           
=======================================
  Files          12       12           
  Lines        3069     3069           
=======================================
  Hits         2410     2410           
  Misses        659      659           

Impacted Files	Coverage Δ
python-package/xgboost/dask.py	78.95% <100.00%> (ø)

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[trivialfis]

So list arguments are not used?

[RAMitchell]

I think requiring explicit kwargs is safer as inputs can't be unintentionally used for the wrong argument.

[trivialfis]

LGTM.