* Add group_key property to Notification type

* Add documentation for new Grouped Notifications

* BREAKING: Move NotificationType to its own class

* BREAKING: Add NotificationTypes MODERATION_WARNING+SEVERED_RELATIONSHIPS

Breaking in case library consumers relied on Enum’s ordinal as new
options were added in-between to keep alphabetical order.

* Add NotificationGroup entity

* Add RelationshipSeveranceEvent to Notification and NotificationGroup

* Add AccountWarning to Notification and NotificationGroup

* Add GroupedNotificationsResults entity

* Allow Pageable on single-page return

Circumvents odd case where api/v2/notifications, despite supporting
pagination, returns just a JSONObject and not a JSONArray

See https://github.com/mastodon/mastodon/issues/32594

* Remove nullability after okhttp update

* Add method go get all grouped notifications

* Add method go get a specific grouped notification

* Add method to dismiss a specific notification group

* Add method to get accounts of notifications in group

* Add method to get unread count

* Add still missing notification endpoints to API coverage documentation

* Add Rx implementation