Emit notifications when creating User Task for a given Integration

[marcoandredinis]

User Tasks is a new resource that contains issues about an integration.
For now, User Tasks are only created to register issues related to auto
enrolling EC2 instances.
Other types of issues will be registered in the future.

Users must proactively list User Tasks in order to understand existing
issues.
For now, this can only be done using tctl get user_task.

An UI for this new resource will be created, but users still need to
navigate to that UI to see what issues exist.

This PR creates a new Notification type which is used to explicitly
notify the user that the Integration has pending User Tasks.

When are users notified about pending User Tasks?
When a User Task is created or updated, if its state is OPEN, then a
notification is created.

Given that we can have dozens of UserTasks for a single integration, we
had to ensure only one Notification exists per Integration.
To do that, the current Notification ID is stored in the Integration's
Status field.
This way, when we try to create a new Notification, we ensure the
existing one is deleted to prevent showing two (or more) Notifications
saying "Your integration needs attention".
All of this is protected by a semaphore lock. The DiscoveryService is
highly async and multiple UserTasks can be created in parallel, and the
locks prevents it.

Before merging:

• (WebUI) Create User Tasks UI
• (WebUI) Create Pending User Tasks Integration in Notifications Factory in UI

[bernardjkim]

Not entirely familiar with the notification system. But does the global notification not need to be deleted when the user task is deleted? Do we just want to wait for it to expire instead?

[marcoandredinis]

Not entirely familiar with the notification system. But does the global notification not need to be deleted when the user task is deleted? Do we just want to wait for it to expire instead?

It should but I'm not too worried about it because UserTasks should expire after 10 minutes (twice the PollInterval of teleport.yaml/discovery_service.poll_interval, which defaults to 5 minutes) - thus, the Notifications will also expire after 10 minutes.
We also give no way for users to delete User Tasks from the UI, they can only mark it as Resolved.

I'm hesitant in adding the deletion of the Notification when a given UserTask is deleted, because it would add some complexity and make the system slower.
This complexity comes from deleting the Notification only when there's no other pending UserTask, and the need to update the Integration.Status.PendingUserTaskNotificationExpiration when the Notification we are about to delete was the longest one.

So, we would need to iterate over all UserTasks to check whether we must delete the Integration or update the expiration date.
All of this under the existing Lock for integration: integration_name key.

I would rather not do it for now, but please let me know what you think.