Acknowledge/Unacknowledged alerts

**What the feature is (as Title)**

Acknowledge and unacknowledge alerts

**Value proposition**
When multiple people triage the same alert, it is hard to tell whether someone is already on it. Without acknowledgement, responders duplicate effort, pass the same alert around, or assume silence means nothing is happening. Existing alert state alone does not communicate investigation intent.

Ack and unack in the alerts list adds a lightweight coordination layer on top of existing alerting. Investigators can mark alerts they are handling; teammates see that signal in the same place they already triage. Because acknowledgement only updates workflow metadata, rules keep evaluating, notifications keep firing as configured, and recovery behaviour stays unchanged, so teams get clarity without turning alerting off or muting rules.

**Expected outcome**
Teams can now acknowledge and unacknowledge alerts directly from the alerts list in Stack Management and Observability. Acknowledging marks an alert as under active investigation and updates it with workflow metadata (kibana.alert.workflow_status: "acknowledged"). Unacknowledging clears that state when ownership changes or the investigation is no longer active. This gives teams a shared, visible signal about who is working on what without changing rule behaviour, notifications, or recovery.