By adding Api version to PluginInfo, it becomes easier to check if all
plugins of a Gerrit server got built for the correct Api. This is
especially useful when upgrading servers.
This commit only exposes the Api version through the Api. Showing it in
the default output of the corresponding Ssh list command or in the UI
will happen in follow-up commits.
Change-Id: Id0c459dd104b4582fde1846e7710d6ad0ffc2f93
c3afed6878