fix: ensure auth_cli does not overwrite existing atkeys files

[srieteja]

Describe the bug

While creating new enrollments, the auth_cli can overwrite MPKAM keys file or other enrollment atKeys files.

Steps to reproduce

1. First I 'create an enrollment at the default atKeys filePath at /home/user/.atsign/keys/'
2. Then I 'create another enrollment without providing an atkeys file path
3. This would overwrite the file created in step 1

Expected behavior

AuthCli should not overwrite atKeys files by default. It should check if the provided filePath already exists and only overwrite if the overwrite flag is set to true.