Design validation and testing hook system

**Summary**: Create flexible hook system that allows custom validation and testing scripts to be executed at various stages of deployment strategies.

**Acceptance Criteria**:
- [ ] Hook system architecture with extensible plugin model
- [ ] Pre-deployment, post-deployment, and mid-deployment hook points
- [ ] Hook execution environment and context management
- [ ] Hook parameter passing and environment variable support
- [ ] Hook timeout and error handling mechanisms
- [ ] Integration with existing deployment strategy workflows