Fix build on latest AppleClang

[hcho3]

Currently, the UBJSON parser in Treelite uses std::basic_string_view<std::uint8_t>, and it fails to compile on latest MacOS.

From the release note of XCode 16.3:

The base template for std::char_traits has been removed. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or a custom character type for which you specialized std::char_traits, your code will stop working. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could previously cause unexpected behavior while going undetected.

Example error:

In file included from $SRC_DIR/src/c_api/model_loader.cc:10:
In file included from $BUILD_PREFIX/bin/../include/c++/v1/string:647:
$BUILD_PREFIX/bin/../include/c++/v1/string_view:300:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'
  300 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
$SRC_DIR/src/c_api/model_loader.cc:85:7: note: in instantiation of template class 'std::basic_string_view<unsigned char>' requested here
   85 |       std::basic_string_view<std::uint8_t>{ubjson_str, length}, config_json);
      |       ^

Fix. Use char to represent bytes in the UBJSON string.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.78%. Comparing base (c106644) to head (55659f3).
⚠️ Report is 1 commits behind head on release_4.5.

Additional details and impacted files

@@             Coverage Diff              @@
##           release_4.5     #629   +/-   ##
============================================
  Coverage        84.78%   84.78%           
============================================
  Files               77       77           
  Lines             6756     6756           
  Branches           531      531           
============================================
  Hits              5728     5728           
  Misses            1028     1028           

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