Improve webhook reviewer type definitions

[liby]

• Extract ReviewerState type to avoid duplication between Webhooks and MergeRequests
• Add WebhookReviewerSchema with proper state and re_requested fields
• Add email field to WebhookUserSchema based on actual webhook payloads
• Add missing 'unapproved' state found in production webhooks
• Update changes.reviewers to use WebhookReviewerSchema

References:

• https://docs.gitlab.com/user/project/integrations/webhook_events/#reviewer-state-tracking
• https://docs.gitlab.com/user/project/integrations/webhook_events/#re-request-review-events