Helper methods for deleting file groups with their contents

[evgkirov]

Description

Related issue: #198

Motivated by #196

Groups are unremovable. But the files in the group are. Some times user has a need to remove all files in a group and currently, the only way to do this is to iterate over files and use delete or batch delete method.
This makes code more complex and, potentially, increases number of requests to achieve the goal.

p.s.: Also, consider adding this as a core REST API functionality?

Changing the Uploadcare API is out of scope of this project. Helpers methods, however, have been implemented.

Example usage for Uploadcare:

uploadcare.delete_file_group_with_files("2e481e82-2e39-41be-a5a2-23802c6e341c~2") 

Example usage for FileGroup:

group = uploadcare.file_group("2e481e82-2e39-41be-a5a2-23802c6e341c~2")
group.delete_with_files()

Checklist

• Tests (if applicable)
• Documentation (if applicable)
• Changelog stub (or use conventional commit messages)