Resolve #7502

This pull request includes several changes to the cp, mv, and uucore utilities, focusing on adding Default implementations to various enums and structs.

The selection of default values is based on the values that would be set when no flag are passed in the command line.

Key changes include:

Enhancements to cp utility:

• Added Default implementations for enums ClobberMode, OverwriteMode, ReflinkMode, SparseMode, CopyMode, and struct Attributes. This simplifies the initialization of these types. [1] [2] [3] [4] [5]
• Added a Default implementation for the Options struct, setting default values for all fields.
• Replaced custom initialization logic with the Default trait for ReflinkMode in the Options struct.

Enhancements to mv utility:

• Added a Default implementation for the Options struct, setting default values for all fields.
• Added a Default implementation for the OverwriteMode enum.

Enhancements to uucore library:

• Added a Default implementation for the BackupMode enum and replaced the hardcoded backup suffix with a constant DEFAULT_BACKUP_SUFFIX. [1] [2]
• Added a Default implementation for the UpdateMode enum.