Implement YAML file parsing and merging

**Summary**: Build the core YAML parsing and merging engine that handles environment-specific configuration files and combines them with base configurations.

**Acceptance Criteria**:
- [ ] YAML parser with support for environment-specific file discovery
- [ ] Configuration merging engine with configurable merge strategies
- [ ] Error handling for malformed YAML and merge conflicts
- [ ] Configuration caching and performance optimization
- [ ] Integration with existing azd configuration loading pipeline