feat: Apkam test scenarios documentation

[murali-shris]

- What I did

• added apkam test scenarios in at_client and at_onboarding_cli - How I did it
• added test scenarios in Gherkin file format.
• added TODO for missing assertions in existing tests - How to verify it -n/a Comments:
• @purnimavenkatasubbu wrote the scenarios for at_client.
• I will make the changes in tests/add new tests once @sitaram-kalluri completes the current code refactoring related to naming convention and usability.
• PR currently in draft. will merge the PR after completing the tests.

[murali-shris]

changes pushed to apkam branch in at_client and merged in at_onboarding_cli