🔧 Support multiple `--mappings-file` flags and merge their contents automatically

[httpdss]

🧩 Feature Request

Add support for passing multiple --mappings-file flags to the struct generate command. When more than one is provided, the CLI should merge all the provided mapping files into a single internal structure before processing.

✅ Expected Behavior

Allow users to define reusable and environment-specific mappings like so:

```sh
# Example: Combine shared and environment-specific mappings
struct generate \
  --mappings-file ./mappings/common.yaml \
  --mappings-file ./mappings/${ENVIRONMENT}.yaml \
  file://infrastructure.yaml \
  ./output
```

This would result in all mappings being merged and applied as if they were defined in a single file.

🚀 Why This Is Useful

• Enables clean separation of common vs environment-specific configuration
• Greatly improves CI/CD automation, as shown in the example
• Promotes DRY (Don't Repeat Yourself) principles

🛠️ Notes for Implementation

• Preserve order of application: first file should be overridden by the second in case of key conflicts
• Validate merged YAML structure before continuing with generation