Add support for async mappings returning CompletableFuture.

[mvh77]

Spring supports returning CompletableFutures for mappings marked with @async. Should be pretty similar to #297.

[mvh77]

It's actually better if the inspected type is the more generic CompletionStage. I'll take a look at this later today.

[bnasslahsen]

@mvh77, can you please provide more informations about your request according to the contribution guide when Using GitHub Issues:

• https://github.com/springdoc/springdoc-openapi/blob/master/CONTRIBUTING.adoc

[mvh77]

Hi! I created a PR here #372