`ServerSentEvent` should implement `equals()` and `hashCode()`

[jelmerk]

ServerSentEvent is an immutable class but does not implement equals and hashCode.

So something like this does not work:

        StepVerifier.create(responseBodyFlux)
            .expectNext(ServerSentEvent.builder(ChatResponseMessage("And")).build())

And instead it forces you to write something like this:

        StepVerifier.create(responseBodyFlux)
            .consumeNextWith { event -> assertThat(event.data()?.text).isEqualTo("And") }

which is both inconvenient and unexpected.