SSE connection presents some instabilities

While using the TOCK React Kit in production with SSE enabled, cases of the SSE connection either disappearing or multiplying occurred sporadically. The current EventSource-based implementation is particularly hard to debug, due to the lack of information on errors or retries (the error event notably is a basic object with no specific data). EventSource is also limited, preventing us from passing on custom headers or a body with the stream request.

A previous PR attempted to replace EventSource with the @microsoft/fetch-event-source package, but this implementation caused other issues (see #185). I believe we could try a custom fetch-based implementation using a well-maintained parsing library like eventsource-parser.

Start by locating the current EventSource-based SSE connection entry point in the React Kit and review issue #185 before changing the approach. Evaluate a fetch-based implementation with eventsource-parser, preserving the stream behavior while exposing error and retry details and allowing custom headers or a request body. Done means the intermittent disappearance or multiplication is addressed without reintroducing the issues from #185.