Implement a switch manager service to simplify and consolidate switch value checking

Problem/Motivation

I think it's largely gonna be a convenience interface for working with the switches. Most plugins have a Plugin Manager class that might be pretty similar, but since these are created as config entities they don't really create one since they use the Entity Type Manager instead.
So basically we'll want to have a class that's going to handle some of that for us and will likely grow over time to ease the DX of checking and working with the switches. That'll need to inject the EntityTypeManager for sure and that'll handle some of the interfacing logic for loading and checking statuses on a switch entity.

Proposed resolution

Remaining tasks

class SwitchManagerInterface {

  /**
   * Loads a switch instance.
   *
   * @param string $switch_id
   *    The switch machine name.
   *
   * @return SwitchEntity
   *   The loaded and configured switch entity.
   */
  public function getSwitch($switch_id);

  /**
   * Returns if a switch is enabled.
   *
   * @param string $switch_id
   *    The switch machine name.
   *
   * @return bool
   *   The activation status for the specified switch entity.
   */
  public function isEnabled($switch_id);

}

User interface changes

API changes

Data model changes