Add a generic AddPlugin option to configure the scheduler

[liu-cong]

What would you like to be added:

An AddPlugin implemented as a type ConfigOption func(*SchedulerConfig) that uses reflection to check which extension points the plugin implements, and add those extension points to the config.

This should also deprecate any existing helper functions to add plugins, such as AddPrefixPlugin

Why is this needed:

This is a follow up to the discussion in #768 (review)

[liu-cong]

/assign

[nirrozenbaum]

/assign
(coordinated with @liu-cong)