feat: Add backup list functionality

[randreev1321]

Changes:

• Added backup listing feature on storage and job interfaces.
• Implemented this feature in NFS, SMB, SFTP, local and webdav storage modules.
• Applied this functionality in file, database, and external jobs.
• Initiated use of backup listing in cmd handler.
• Introduced new error ErrExecution to denote execution errors.
• Improved sorting functionality in local, SMB and NFS storage's DeleteOldBackups methods.
• Renamed NFS module file to nfs_v3.go for clarity.
• Fixed file path configuration issue in local and webdav storage.
• Fixed delete old backups functionality for NFS storage.
• Tidied up minor formatting and variable naming issues.