Add event filter for notifications

[RemiRigal]

This PR implements enhancements requested in #36.

Notification configurations can now have an additional events array that allows for getting notifications only for a set of events.

Example configurations:

plexautolanguages:

  [...]

  notifications:
    enable: true
    apprise_configs:
      - urls:
          - "tgram://bottoken/ChatID"
        users:
          - "MyUser3"
        events:
          - "play_or_activity"
      - urls:
          - "gotify://hostname/token"
          - "tgram://bottoken/ChatID"
        events:
          - "scheduler"
          - "new_episode"

Valid events are:

• play_or_activity
• new_episode
• updated_episode
• scheduler