SLK-110843 feat(webhook): add Authorization header support for webhook outputs

[lionelresnik]

Summary

• Added Headers field to WebhookOutput struct to support custom HTTP headers
• Modified Send() method to use http.NewRequest instead of http.Post to allow setting headers
• Updated buildWebhookOutput in router/builders.go to pass headers from OutputSettings
• Added unit tests for webhook authentication scenarios (Bearer token, Basic auth)

Why

This enables webhook authentication (Bearer Token and Basic Authentication) for the Notifications pipeline, matching the existing Log Management webhook authentication feature (SLK-84531).

Test plan

• Unit tests added for Bearer token auth
• Unit tests added for Basic auth
• Unit tests added for no auth (backward compatibility)
• Unit tests added for error scenarios

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