Support for plugins downloaded from a different source

[clarsonneur]

jplugins should support installing plugins from a different source so that it can check and update the plugin from that source.

ex: Veracode jenkins plugins

This page explains how to download a version from their site ie:
https://tools.veracode.com/integrations/Jenkins/bin/veracode-jenkins-plugin.hpi

We can define it in the features.lst as follow:

plugin:veracode-jenkins-plugin::https://tools.veracode.com/integrations/Jenkins/bin/veracode-jenkins-plugin.hpi

But It could be incompatible with the way this lst file is already managed. To verify.

A better approach could be to use a features.yaml. This approach requires a little more development.
But that way we can configure jplugins features better.

plugins:
  veracode-jenkins-plugin:
     source: https://tools.veracode.com/integrations/Jenkins/bin/veracode-jenkins-plugin.hpi
     source-type: download # default to `updates-repo`
     version: # default to latest