SNOW-802269 To Add reverse isnull conv unixtimestamp

[sfc-gh-sjayabalan]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Added following functionalities Reverse function isnull conv unixtimestamp

2. Fill out the following pre-review checklist:

   • [ yes] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding a new telemetry message
   • [ ] I am adding new credentials
   • [ ] I am adding a new dependency

3. Please describe how your code solves the related issue.

   Replace this with a short description of how your code change solves the related issue.

Pre-review checklist

(For Snowflake employees)

• [ ] This change has passed precommit
• [ ] I have reviewed code coverage report for my PR in (Sonarqube)