Add support for command-based health checks in addition to HTTP-based checks:
- New health_check_type field supporting 'http' and 'cmd' values
- New health_check_command field with strict regex validation
- Updated allowedFields in create_application and update_by_uuid endpoints
- Validation rules include max 1000 characters and safe character whitelist
- Added feature tests for health check API endpoints
- Added unit tests for GithubAppPolicy and SharedEnvironmentVariablePolicy
