Centralize Server Sent Event utility methods

[bclozel]

I've noticed a bit of code duplication for Server Sent event handling between MVC and Reactive (functional and annotation variants). This is not much but we should move this code into a shared SseUtils class.

[arnabnandy7]

@bclozel if possible please assign me, I'm interested in contributing.

[bclozel]

Thanks but this is already assigned and I have the changes ready