Support XGBoost UBJSON

[hcho3]

Closes #573

Use https://github.com/nlohmann/json to parse UBJSON as it has the following advantages:

• Idiomatic modern C++
• Available on Conda-forge
• Offers SAX-style parser API, so that we can re-use substantial amount of code from XGBoost JSON parser.

[hcho3]

@trivialfis FYI, I managed to implement the UBJSON support without too much work. It reuses the event-based parser we use for parsing XGBoost JSON models.

[codecov]

Codecov Report

Attention: Patch coverage is 78.58974% with 167 lines in your changes missing coverage. Please review.

Project coverage is 84.77%. Comparing base (d8f12bc) to head (31d80c0).
Report is 6 commits behind head on mainline.

Files with missing lines	Patch %	Lines
...el_loader/detail/xgboost_json/delegated_handler.cc	77.98%	118 Missing ⚠️
...c/model_loader/detail/xgboost_json/sax_adapters.cc	55.38%	29 Missing ⚠️
src/c_api/model_loader.cc	70.96%	9 Missing ⚠️
python/treelite/frontend.py	90.47%	4 Missing ⚠️
src/model_loader/detail/xgboost.cc	78.94%	4 Missing ⚠️
...del_loader/detail/xgboost_json/delegated_handler.h	87.50%	2 Missing ⚠️
src/model_loader/lightgbm.cc	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           mainline     #578      +/-   ##
============================================
- Coverage     84.92%   84.77%   -0.16%     
============================================
  Files            71       75       +4     
  Lines          6347     6547     +200     
  Branches        521      528       +7     
============================================
+ Hits           5390     5550     +160     
- Misses          957      997      +40     

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[trivialfis]

Cool, the nlohmann JSON is excellent, glad that it can be directly used here.