adding new attribute classes and converters for comma separated string and timedelta in months

[vnktshr21]

• [x] This contribution adheres to CONTRIBUTING.md.
• [ ] ~~I've updated CHANGELOG.md if applicable.~~
  • Not updating since this is an internal change
• [x] I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Added new attribute classes for
  • Comma separated string, but exposed in python API as list of strings
  • Time interval in months, but exposed in python API as hightime.timedelta
• Added new nifake attributes for these classes and unit tests validating their behavior
• Added converters for
  • list of strings to comma separated string
  • timedelta to months
    • Using the same 365/12=30.4167 conversion that was used previously
    • This is to cater to our C API returning interval values in months and us standardizing on using hightime.timedelta type for intervals in python API
• Converters for the opposite side were already implemented
• Added tests for newly added converters

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

Build passes.