Explore removal of Apache's httpclient dependency from flow-server

[knoobie]

Describe your motivation

Currently vaadin-flow (flow-server) has a dependency to the httpclient (4.5.13) from Apache. The only class that needs it in flow-server DefaultFileDownloader which is only used on build / development time.

Describe the solution you'd like

Remove the dependency to Apache's httpclient by e.g. using Java 9's HttpClient or by moving of the whole class and its callers to the dedicated maven / gradle base plugin to reduce the dependency on runtime / production mode.