Support generation_kwargs for Agent

[jdb78]

Is your feature request related to a problem? Please describe.
One cannot pass model parameters to the Agent dynamically.

Describe the solution you'd like
Add generation_kwargs to Agent class's run and run_async methods, which is passed to the chat_generator. Currently, only the streaming_callback is supported.

Describe alternatives you've considered
Initializing the pipeline everytime before it is being used