AutoConfiguration support for Data Mongo ReactiveGridFsTemplate

[hantsy]

The latest Spring Data Mongo has brought Reactive support for GridFs, but Spring Boot 2.2.0.M1
is lack of an AutoConfiguration class to activate it, I have to configure it manually at the moment.

https://github.com/hantsy/spring-reactive-sample/blob/master/boot-data-mongo-gridfs/src/main/java/com/example/demo/DemoApplication.java#L35-L43

[ayudovin]

@wilkinsona, I would like to work on it. Please let me know if I can start working on it?

[philwebb]

@ayudovin Much appreciated. The issue isn't assigned and it doesn't look like it should be too involved so please go head.

[mbhave]

Closing in favor of PR #16467.