feat: Add #insert directive for simple content inclusion

[agonzalezrh]

Summary

This PR adds a new directive that provides a clean way to include content without applying merge strategies.

Problem

Users needed a way to include content from other YAML files without the complex merge strategies that applies. The existing directive applies all merge strategies, which can be too aggressive for simple content inclusion.

Solution

Added a new directive that:

• Includes content without applying merge strategies
• Processes directives within inserted files
• Performs selective merging for section (only and fields)
• Uses simple overwrite for non- fields

Changes

• includes.go: Added struct, function, and function
• merge.go: Updated merge logic to handle directives with selective merging
• merge_test.go: Added test case for parsing include directives with options

Usage

# Simple content inclusion without merge strategies
#insert /path/to/file.yaml

# Still works as before with merge strategies
#include /path/to/file.yaml

Benefits

• Clean separation: vs are clearly different
• Backward compatible: All existing directives continue to work
• Simple and intuitive: means "just add this content"
• Flexible: Processes includes within inserted files but without merge strategies

Testing

• ✅ Preserves local values (asset_uuid, components)
• ✅ Adds content from inserted files (sandboxes, deployer)
• ✅ Processes #include directives within inserted files
• ✅ Selective merging for meta section
• ✅ Simple overwrite for other fields