Add response headers to WebOutput

[andimarek]

Sometimes you want to add additional custom response headers to the HTTP Response. WebOutput should support this.

[rstoyanchev]

Indeed WebOutput would be a good place to expose response HttpHeaders for customization, symmetrical to WebInput. However on the WebMvc.fn side there is a shortcoming that doesn't allow this.

I've created spring-projects/spring-framework#25828. In the mean time we can proceed with adding HttpHeaders to WebOutput. Initially it will only work on WebFlux.fn and once the referenced issue is resolved we'll support it for WebMvc.fn as well.