Enrollment Plugins should be selectable for enrollment flows

Enrollment flow plugins can extend and adjust the enrollment flow in very specialistic ways. When hosting multiple COs, a plugin might be required for a specific enrollment flow of one CO, but certainly not for the other.

Examples are: adding additional information into the flow, showing additional input and/or verification screens, etc.

Enrollment flows should have the option of selecting which available plugins are to be ignored. The CoPetitionsController can then ignore that plugin when executing/dispatching the enrollemt step.

This prevents plugins from running even if they add nothing to the current flow (speeding up the process significantly) and prevents developers from having to jump through complicated loops to determine if the plugin actually needs to do something for a specific flow.