Add file streaming for uploads/downloads

[william746a]

Motivation:

Explain here the context, and why you're making that change, what is the problem you're trying to solve.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md
Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[tsegismont]

Thanks for sharing this, @william746a

I looked only at the vertx-web changes so far, and I would prefer to avoid all the changes in RoutingContext API.

While we had file upload related methods at the RoutingContext level for ages, we added the io.vertx.ext.web.RoutingContext#body in relatively recent versions of Vert.x. The new method to retrieve a streamed body file should be there imo.

Also, can you please split the PR into two commits to make it easier to review? One for the vertx-web changes and another for vertx-web-api? Thanks

[william746a]

@tsegismont you got it, I'll get started on round 2 this week.