feat: add OpenAI-compatible AI provider plugin with SSE streaming support

- Implemented the OpenAI-compatible AI provider plugin, including configuration, chat, and chat_stream functionalities.
- Added support for SSE streaming and tool calls.
- Integrated Boost.JSON for JSON handling.
- Created CMake configuration for the plugin.
- Added error handling and logging throughout the plugin.

plugins_upper/openai/CMakeLists.txt

@@ -0,0 +1,20 @@
+# ============================================================
+# plugin-openai — OpenAI 兼容 AI 服务 / OpenAI-compatible AI service
+# ============================================================
+
+find_package(Boost REQUIRED CONFIG)
+
+add_library(plugin-openai SHARED
+    src/openai_plugin.cpp
+)
+
+target_link_libraries(plugin-openai PRIVATE dstalk)
+
+# Boost.JSON (header-only)
+target_link_libraries(plugin-openai PRIVATE boost::boost dstalk_boost_config)
+
+set_target_properties(plugin-openai PROPERTIES
+    PREFIX ""
+    LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/plugins
+    RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/plugins
+)