[Multi-Step Update] Syncer Enhancement

[ervcz]

Description

Enhance syncer to handle multi-package responses and reconstruct prerequisite relationships.

Tasks

• Detect syncer vs regular client in requests
• Generate extended Omaha response for capable syncers
• Include prerequisite metadata in response
• Implement version-based relationship reconstruction
• Validate complete chains before channel updates
• Handle legacy syncers with fallback to single-package mode

Extended Omaha Format

<package name="3374.2.0/flatcar_production_update.gz">
  <update_type>prerequisite</update_type>
  <version>3374.2.0</version>
</package>
<package name="3500.0.0/flatcar_production_update.gz">
  <update_type>target</update_type>
  <version>3500.0.0</version>
  <requires_version>3374.2.0</requires_version>
</package>