feat: Added support for streaming in chat as a run_manager

[MateuszOssGit]

Added support for streaming in chat as a run_manager

Before:

chat = ChatWatsonx(
    url=credentials["url"],
    apikey=credentials["apikey"],
    model_id=model_id,
    project_id=project_id, 
    streaming=True
)
chat.invoke("hello")

# Out
 AttributeError: 'ChatGenerationChunk' object has no attribute 'content'

After:

chat = ChatWatsonx(
    url=credentials["url"],
    apikey=credentials["apikey"],
    model_id=model_id,
    project_id=project_id, 
    streaming=True
)
chat.invoke("hello")

# Out
AIMessage(content='Hello. How can I help you?', response_metadata={'finish_reason': 'eos_token'}, id='sample_id')

[Mateusz-Switala]

LGTM