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/context/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_library(plugin-context SHARED src/context_plugin.cpp)
+
+target_link_libraries(plugin-context PRIVATE dstalk)
+
+set_target_properties(plugin-context PROPERTIES
+    PREFIX ""
+    LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"
+    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/plugins"
+)