Refactor default labels into a reusable module

This MR extracts the default labels logic from the AWS fleeting module into a new reusable internal module.

The changes:

• Creates a new module modules/internal/default_labels to centralize label management
• Modifies the AWS fleeting module to use this new common module (will update this branch to include others if the pattern is approved)
• Simplifies label handling by standardizing the merge pattern across modules