Add Opsgenie alias field for alert deduplication

[cairon-ab]

Summary

Set the alias field on Opsgenie alerts using a SHA-256 hash of the involved object's Kind, Namespace, Name, and event Reason. This allows Opsgenie to deduplicate repeated alerts for the same source instead of creating new pages for each firing.

Changes

• Added Alias field to OpsgenieAlert struct with json:"alias,omitempty"
• Added generateOpsgenieAlias() function that creates a deterministic SHA-256 hash from the event's InvolvedObject.Kind, InvolvedObject.Namespace, InvolvedObject.Name, and Reason
• Wired the alias into Post() so every alert carries a stable alias
• Added comprehensive tests: TestOpsgenie_PostAlias (verifies alias in HTTP payload across multiple scenarios) and TestGenerateOpsgenieAlias (determinism, length, uniqueness)

Why

Currently, Opsgenie creates a new alert for every notification event, even when it's for the same object and reason. The Opsgenie Alert API supports an alias field that acts as a deduplication key — alerts with the same alias are grouped as a single incident.

By hashing Kind/Namespace/Name/Reason, we ensure:

• Determinism: Same event source always produces the same alias
• Deduplication: Repeated firings update the existing alert instead of paging again
• Differentiation: Different reasons (e.g. ReconciliationFailed vs HealthCheckFailed) create separate alerts
• Size safety: The 64-char hex hash stays well within Opsgenie's 512-char alias limit

Fixes #460

[stefanprodan]

This will cause alerts from multiple clusters to aggregate under the same incident since all clusters have Kustomization/flux-system/flux-system, which is a major breaking change. Adding the Alert Provider UID to the checksum would ensure each cluster gets a dedicated incident.