Updated few nirfsg properties to have string and interval datatypes consistent with other APIs in repository

[vnktshr21]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Updated external_calibration_recommended_interval property to return a hightime.timedelta object instead of months as an int, aligning with other APIs in the repository
• Updated below properties to be of list of strings type instead of them returning a comma separated list of string values
  • available_paths
  • available_ports
  • fixed_group_delay_across_ports
  • group_capabilities
  • supported_instrument_models

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

None

What testing has been done?

System tests executed successfully with changes to attributes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.08%. Comparing base (442e169) to head (e9ea4f3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2152   +/-   ##
=======================================
  Coverage   90.07%   90.08%           
=======================================
  Files          71       71           
  Lines       18664    18664           
=======================================
+ Hits        16812    16813    +1     
+ Misses       1852     1851    -1     

Flag	Coverage Δ
codegenunittests	84.44% <ø> (ø)
nidcpowersystemtests	94.65% <ø> (+0.04%)	⬆️
nidcpowerunittests	89.53% <ø> (ø)
nidigitalsystemtests	92.26% <ø> (ø)
nidigitalunittests	68.44% <ø> (ø)
nidmmsystemtests	92.72% <ø> (ø)
nifakeunittests	85.49% <ø> (ø)
nifgensystemtests	94.61% <ø> (ø)
nimodinstsystemtests	73.85% <ø> (ø)
nimodinstunittests	94.20% <ø> (ø)
nirfsgsystemtests	82.40% <100.00%> (ø)
niscopesystemtests	92.94% <ø> (ø)
niscopeunittests	43.20% <ø> (ø)
nisesystemtests	91.50% <ø> (ø)
niswitchsystemtests	82.03% <ø> (ø)
nitclksystemtests	94.87% <ø> (ø)
nitclkunittests	98.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
generated/nirfsg/nirfsg/session.py	90.22% <100.00%> (ø)

... and 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 442e169...e9ea4f3. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

Requested changes were made.