UploaderService: support auto-installing uploaders given their protocol

The idea is that uploaders for specific protocols follow the convention
that they are contained in 'ij-updater-<protocol>-<version>.jar' files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: Add a method to find upload protocols given files

Every uploading mechanism will need to verify that the uploaders
required to upload a certain set of files are available. This method
helps.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing): use the newly introduced API to auto-install uploaders

There was little use in hiding that functionality in the Swing-specific
frontend of the updater to begin with.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add an integration test for the uploader auto-install

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>