[JENKINS-34833] - Installation Wizard: Allow altering the list of suggested...

[JENKINS-34833] - Installation Wizard: Allow altering the list of suggested plugins from update sites (#2348)

* [JENKINS-34833] - Installation Wizard: Allow altering the list of succested plugins from update sites

In our company we provide a custom package of Jenkins, which provides a customized set of plugins (open-source and closed-source ones). These plugins come from the custom update site.

We would like to provide custom suggestions to users of our Jenkins packages when they install Jenkins. For such purpose we would like to make the plugin suggestions list overridable by UpdateSite extensions in Jenkins.

* [JENKINS-34833] - Fix the JS Unit test (code from @kzantow)

* [JENKINS-34833] - Unit tests for SetupWizard

* [JENKINS-34833] - Address comment regarding the error propagation for the unknown version

* [JENKINS-34833] - SetupWizard::getPlatformPluginList() is not a part of the public API

* [JENKINS-34833] - Address comments from @orrc