Description
The Chat::submit() function didn't support streaming. I have added support for this through:

1. Creating the AccumulatingStreamResult wrapper which allows callback chaining and accumulates the full message (callbacks are fired once Generator in inner StreamResult is exhausted, aka when stream has completed)
2. Wrap StreamResult classes with this wrapper inside the Chat::submit() function
3. Create a small callback function which updates the Storage with a new AssistantMessage built from the accumulated message (as non-stream responses are also automatically added to storage in Chat::submit())

Other changes:

• Added StreamableStoreInterface which indicates a Store implementation supports streaming. Currently added to all stores except SessionStorage (due to inherent issues with headers being already sent; deferred updates through callbacks go against nature of header/session lifecycles in Symfony; discussed with @Guikingone)
• Fixed bug where metadata was not persisted in Chat::submit() (metadata was not transferred to new AssistantMessage we created)
• Added tests for streaming and Chat::submit() combination