Add SSE Support to RestClient

[asaikali]

In Spring WebClient is the only supported way to consume SSE streams. However, using WebClient introduce a reactive streams increasing application complexity.

Ideally RestClient can support the consumption of SSE streams allowing developers to keep codebases simple.

Spring offers reactive and non-reactive facilities for producing SSE events. Adding the ability to consume SSE streams in a reactive and non-reactive ways bring the server side and client side experience to parity.

[tzolov]

I believe this is related: #34290