fix: Add .atKeys extension to the custom file path

[sitaram-kalluri]

- What I did

• Fixes issue: https://github.com/atsign-foundation/at_libraries/issues/629

- How I did it

• In the atKeys file path, if the file name doesn’t end with the 'atKeys' extension, the problem occurs because "atKeys" is appended to the path rather than being set as the file extension.
• Replaced path.join method with path.setExtension to fix the issue.

- How to verify it

• Tested manually after the fix and below is log snippet:

  sitaram@sitaram-ThinkPad-E14:~/IdeaProjects/atsign/core/at_libraries/packages/at_onboarding_cli$ dart bin/activate_cli.dart enroll -s ABC123 -p demo2 -d local -n "buzz:rw" -a @sitaram -r vip.ve.atsign.zone -k ~/keys/akam_keys/@sitaram_1_newkeys
  Submitting enrollment request
  Enrollment ID: 434a7047-18df-4ea2-9c24-bc0306adce2b
  Waiting for approval; will check every 10 seconds
  Checking ...  not approved. Will retry in 10 seconds
  Checking ...  not approved. Will retry in 10 seconds
  Checking ...  approved.
  Creating atKeys file
  [Success] Your .atKeys file saved at /home/sitaram/keys/akam_keys/@sitaram_1_newkeys.atKeys

• Added unit test to assert the change.