feat(core): add `EventSource` protocol

[ajndkr]

Description

Fixes #121 Fixes #112

This PR adds the EventSource protocol for streaming LLM output. We still respect the text and JSON streaming modes but all events will now stream as follows:

data: <llm_token_1>

data: <llm_token_2>

This also allows the possibility of implementing named events for special use cases:

event: <event_a>
data: <llm_token_1>

event: <event_b>
data: <llm_token_2>

Finally, the StreamingResponse also improves error handling by sending a 500 error event when chain execution fails (Fixes #146)

Changelog:

• ➕ added sse-starlette dependency
• ♻️ refactored StreamingResponse class and base callback handlers