Document model_type='xgboost_json' in FIL

[hcho3]

Closes #3625. Treelite already supports the XGBoost JSON format, so we just need to expose the capability to the FIL loading function too.

Also add model_type='xgboost_json' to the FIL testing matrix.

[hcho3]

CodeCov failed mysteriously... https://gpuci.gpuopenanalytics.com/job/rapidsai/job/gpuci/job/cuml/job/prb/job/cuml-gpu-test/CUDA=10.1,GPU_LABEL=gpu,OS=ubuntu16.04,PYTHON=3.7/818/console

[hcho3]

rerun tests

[hcho3]

rerun tests

[dantegd]

@gpucibot merge

[hcho3]

rerun tests

[hcho3]

Rerun tests

[hcho3]

The Naive Bayes tests keep failing because the dataset download times out.

[hcho3]

rerun tests

[codecov-io]

Codecov Report

Merging #3633 (1bd0a06) into branch-0.19 (14bd6c1) will increase coverage by 0.73%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #3633      +/-   ##
===============================================
+ Coverage        80.72%   81.46%   +0.73%     
===============================================
  Files              228      228              
  Lines            17629    18087     +458     
===============================================
+ Hits             14231    14734     +503     
+ Misses            3398     3353      -45     

Flag	Coverage Δ
dask	45.06% <ø> (+0.10%)	⬆️
non-dask	73.91% <ø> (+0.96%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
python/cuml/fil/fil.pyx	93.48% <ø> (+1.65%)	⬆️
python/cuml/dask/solvers/cd.py	100.00% <0.00%> (ø)
python/cuml/pipeline/__init__.py	100.00% <0.00%> (ø)
python/cuml/common/numba_utils.py	0.00% <0.00%> (ø)
python/cuml/neighbors/__init__.py	100.00% <0.00%> (ø)
python/cuml/internals/global_settings.py	100.00% <0.00%> (ø)
python/cuml/dask/preprocessing/encoders.py	100.00% <0.00%> (ø)
python/cuml/metrics/_ranking.py	98.57% <0.00%> (+0.02%)	⬆️
python/cuml/preprocessing/encoders.py	95.10% <0.00%> (+0.02%)	⬆️
python/cuml/common/array_descriptor.py	98.24% <0.00%> (+0.03%)	⬆️
... and 42 more

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