feat: add logic to improve atKeys file naming

[srieteja]

Closes https://github.com/atsign-foundation/at_libraries/issues/601 - What I did

• validate if the atKeysFilePath provided by the user is a valid file (or) a directory and take necessary action before creating atKeys file.

- How I did it

• If the atKeysFilePath provided is a directory, create the atKeys file inside that directory
• If the atKeysFilePath provided is a file but misses the .atKeys suffix, append one

- How to verify it

• tests on the way

- Description for the changelog

feat: add logic to improve atKeys file naming

[srieteja]

Closing this as the same issue is solved in https://github.com/atsign-foundation/at_libraries/issues/629