[c++] include <cstdint> wherever uint8_t is used

[jameslamb]

Earlier today, I got an email from Prof. Brian Ripley (from CRAN):

subject: CRAN packages missing <cstint> header

body: Current snapshots of g++ 15 are complaining that you have omitted the header . Please follow the advice in the log at your next update. (We would contact you again if this persists when a release date for GCC 15 is known, but they tend to give only a week's notice.)

Linking to these logs for a build of lightgbm with gcc-15: https://www.stats.ox.ac.uk/pub/bdr/gcc15/lightgbm.out

It contains compilation errors like this:

io/json11.cpp: In function 'void json11_internal_lightgbm::dump(const std::string&, std::string*)':
io/json11.cpp:93:28: error: 'uint8_t' does not name a type
   93 |     } else if (static_cast<uint8_t>(ch) <= 0x1f) {
      |                            ^~~~~~~
io/json11.cpp:26:1: note: 'uint8_t' is defined in header '<cstdint>'; this is probably fixable by adding '#include <cstdint>'

From the gcc docs (https://gcc.gnu.org/gcc-15/porting_to.html):

Some C++ Standard Library headers have been changed to no longer include other headers that were being used internally by the library. As such, C++ programs that used standard library components without including the right headers will no longer compile.

In particular, the following header is used less widely within libstdc++ and may need to be included explicitly when compiling with GCC 15:

• <cstdint> (for std::int8_t, std::int32_t , etc.)

There are more notes on this at https://gcc.gnu.org/gcc-15/porting_to.html

Notes for Reviewers

CI job?

I'm not sure how to get a development version of GCC 15 here in CI, to confirm that this is working. https://r-hub.github.io/containers/, for example, does not have a gcc-15 image yet.

If this PR passes all CI (which tests a wide range of compilers and operating systems), I think we should merge it and include it in the next release. Hopefully that will be sufficient.

We can test against gcc-15 as a separate effort.