Add log callback support to Logger class

[nbarbier-amira]

Adds callback-based logging to the Logger class, which allows applications embedding Triton to hook into the logging system and forward messages to their own logging infrastructure.

Motivation

Currently the only way to capture Triton logs programmatically is to redirect to a file and parse it. This is awkward for applications that want to integrate Triton logs with their existing logging setup (structured logging, log aggregation services, etc). See triton-inference-server/server#8304.

Changes

• New LogCallbackFn typedef and SetLogCallback() method on Logger
• Callback receives level, filename, line, and message
• Option to replace default logging or run callback alongside it
• LOG_SET_CALLBACK macro follows existing pattern

Example

void my_logger(Logger::Level level, const char* file, int line, 
               const char* msg, void* ctx) {
    // forward to spdlog, log4cxx, etc
}

gLogger_.SetLogCallback(my_logger, nullptr, true);  // replace default

Notes

This is the foundation for adding TRITONSERVER_ServerOptionsSetLogCallback to the C API in triton-core (separate PR).

Backward compatible - default behavior unchanged when no callback is registered.