feat(integrations): async Openlayer callback handler

[gustavocidornelas]

Pull Request

Summary

Implements the AsyncCallbackHandler interface from LangChain, thus enabling tracing async workflows from LangChain/LangGraph.

Changes

• Moved shared logic from the sync and async handlers to the OpenlayerHandlerMixin class.
• Both OpenlayerHandler and AsyncOpenlayerHandler inherit from the mixin.
• AsyncOpenlayerHandler overwrites the create_step/end_step logic to avoid using context variables (as there are issues in async environments). Otherwise, it uses the methods from the mixin as much as possible.

Context

POC request.

Testing

• Manual testing