feat: Support external repository references for account customizations

[chakeega]

Summary

This PR enables the account_customizations_name field to reference external Git repositories, allowing independent versioning and release cycles per account customization without requiring all customizations to live in the aft-account-customizations repository.

Changes

Buildspec Updates (aft-account-customizations-terraform.yml)

• Added is_repo_reference() to detect external repository references
• Added parse_repo_reference() with support for three formats:
  • repo::owner/repo - simple prefix format
  • https://github.com/owner/repo - HTTPS URLs
  • git@github.com:owner/repo.git - SSH URLs
• Added @ref suffix support for pinning to branches/tags
• Added clone_external_repo() using existing VCS provider credentials
• Added validate_customization_structure() to verify cloned repos have required terraform/ and api_helpers/ directories
• Restructured install phase to check for repo references before folder lookup

Documentation

• Updated README with usage examples and format documentation

Usage Examples

# External repo with version tag
account_customizations_name = "repo::acme/sandbox-customizations@v1.0.0"

# HTTPS URL
account_customizations_name = "https://github.com/acme/sandbox-customizations@main"

# SSH URL
account_customizations_name = "git@github.com:acme/sandbox-customizations.git@v2.0"

# Existing folder-based (unchanged)
account_customizations_name = "sandbox"

Backward Compatibility

Existing folder-based customizations continue to work unchanged. Values without repo::, https://, or git@ prefixes are treated as folder names.