Provide access to non-standard status codes from ExchangeResult

[wilkinsona]

Similar to #22872 and #23547 it would be useful for ExchangeResult to provide support for and access to non-standard status codes as an int.

[wilkinsona]

To illustrate the current situation, the following test will fail:

WebTestClient
	.bindToRouterFunction(
		RouterFunctions.route(GET("/customStatus"), (req) -> ServerResponse.status(600).build()))
	.build().get().uri("/customStatus").exchange().expectStatus().isEqualTo(600);

The status is actually 200 due to the null returned by HttpStatus.resolve(int) then being mapped to HttpStatus.OK.

Edit: looks like @poutsma is day ahead of me for this part of the problem (#23599). Although, that said, an exception is now thrown for statuses < 100 or >= 600. I've commented on #23599 with more details.