Webhooks: support flow process lifecycle events and connect those to Slack alerts

[zaychenko-sergei]

Currently the flow system generates the following important process state lifecycle events via the outbox:

#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum FlowProcessLifecycleMessage {
    FailureRegistered(FlowProcessFailureRegisteredMessage),
    EffectiveStateChanged(FlowProcessEffectiveStateChangedMessage),
    TriggerAutoStopped(FlowProcessTriggerAutoStoppedMessage),
}

Apart from e-mailing, we should support these as webhook subscriptions.

Requirements:

• creating a subscription at:
  - dataset level
  - account level (applies to all accessible datasets)
  - system level (for admins - applies to all datasets at all)
• allow watching dataset flows only for now (don't do crazy things like webhook for a failure of webhook):
  - ingest / transform
  - compact / reset
• new webhook event types: FLOW.PROCESS.FAILED, FLOW.PROCESS.STATECHANGE, FLOW.PROCESS.AUTOSTOP
  (we may start just from FLOW.PROCESS.STATECHANGE, it should be enough to build alerting)
• UI to manage the subscriptions:
  - extend current dataset settings for webhooks
  - account-level
  - system-level for admins
• learn to display these flows in UI
• propagate the related initiating flow information:
  - either extend logic of flow sensors to dispatch failures, not just successes like now
  - or enrich and utilize flow_id from the outbox events, so that a proper activation cause could be built
• prototype Slack application that handles these new webhooks
• connect Slack application to engineering-alerts channel in Kamu workspace