Migrate from `eventsource_client` in `fuel-core-client`

[Br1ght0ne]

eventsource_client is used by fuel-core-client to handle SSE (server-sent events). The crate is not maintained to a high enough standard, and there were cases missing functionality or wrongly implemented standards. I suggest moving to a crate like reqwest-eventsource, which uses reqwest under the hood. reqwest handles all the aforementioned cases correctly, so I expect a crate like this to perform better.

[MitchTurner]

This seems like something we should track, but its hard to know how to prioritize this before understand more deeply the risks of sticking with eventsource_client and the risks and costs and breaking changes with moving to reqwest-eventsource.

For now, I'm going to leave at low priority.