feat: Add Dash0 On Alert Event trigger

[fadhilijuma]

Implements part of #2906.

Summary

• Adds Dash0 onAlertEvent webhook trigger.
• Supports fired, resolved, or both event types with normalization.
• Registers Dash0 webhook handler and merge logic for shared endpoint configuration.
• Adds backend tests, UI mapper/configuration, and generated component docs updates.

Demo Video

• Dash0 components demo

Validation

• go test ./pkg/integrations/dash0 -count=1

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