Currently it is possible to declare used tools for a declarative pipeline, it would be good to have an additional

plugins {
 'extmail',
 'slack',
 '....'
}

section where a pipeline can declare required plugins.

Before the pipeline build starts, the code should check that the plugin is installed and if not halt the build with a notification (like input step) to ask the user if he wants to install the required plugins. If yes is choosen, the user is taken to the pluginmanager with all required plugins preselected.