Enable configuration for custom libpath

[hcho3]

Users can now provide a custom path for libtreelite.so when installing the Python package by using the option system_libtreelite_dir:

pip install -v . --config-settings use_system_libtreelite=True \
   --config-settings system_libtreelite_dir=/opt/treelite/lib

When system_libtreelite_dir is specified, the path_config.py file will be overwritten.

Note: To use system_libtreelite_dir option, users must also specify use_system_libtreelite=True.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4477280) 84.22% compared to head (248b2be) 83.25%.

❗ Current head 248b2be differs from pull request most recent head 0413581. Consider uploading reports for the commit 0413581 to get more accurate results

Files	Patch %	Lines
python/treelite/libpath.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           mainline     #543      +/-   ##
============================================
- Coverage     84.22%   83.25%   -0.97%     
============================================
  Files            70       71       +1     
  Lines          6066     5913     -153     
  Branches        510      510              
============================================
- Hits           5109     4923     -186     
- Misses          957      990      +33     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.