Add coroutine variant of WebExceptionHandler

[earlgrey02]

As in the case of CoWebFilter, I added CoWebExceptionHandler, a coroutine variant of WebExceptionHandler used for global exception handling in Spring WebFlux.

[sdeleuze]

We need to be careful about not going too far in terms of Coroutines duplications of Reactive contrats, but I think I am in favor of that one for 2 reaons:

• That seems indeed pretty close to CoWebFilter use cases which is pretty popular among Kotlin developers I think
• This is not yet reflected in the PR, but CoWebExceptionHandler could introduced the same kind of Coroutines context management as in CoWebFilter.

@rstoyanchev Would you be ok for that?

@earlgrey02 What about refining your PR to provide Coroutines context management like CoWebFilter does?

[rstoyanchev]

If CoWebFilter exists then yes a CoWebExceptionHandler next to it makes sense.

[earlgrey02]

Instead of creating a separate COROUTINE_CONTEXT_ATTRIBUTE for CoWebExceptionHandler, I reused the COROUTINE_CONTEXT_ATTRIBUTE used in the existing CoWebFilter and handler(CoroutineContextAwareHandlerFunction).

[sdeleuze]

Merged and polished, thanks for your contribution.