Add string enums for applicable attributes and method parameters in nirfsg

[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 below attributes and method parameters to be of string enum type instead of plain string, since they would support only fixed set of strings
  • pulse_modulation_source
  • exported_pulse_modulation_event_output_terminal
  • exported_ref_clock_output_terminal
  • lo_source
  • arb_sample_clock_source
  • ref_clock_source
  • trigger_identifier parameter in send_software_edge_trigger method

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

None

What testing has been done?

Build passes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.08%. Comparing base (ee4860e) to head (374e0e3).

Files with missing lines	Patch %	Lines
generated/nirfsg/nirfsg/session.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2150   +/-   ##
=======================================
  Coverage   90.08%   90.08%           
=======================================
  Files          71       71           
  Lines       18644    18646    +2     
=======================================
+ Hits        16796    16798    +2     
  Misses       1848     1848           

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	85.52% <ø> (ø)
nifgensystemtests	94.61% <ø> (ø)
nimodinstsystemtests	73.85% <ø> (ø)
nimodinstunittests	94.20% <ø> (ø)
nirfsgsystemtests	82.40% <93.33%> (-0.03%)	⬇️
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/_library_interpreter.py	75.70% <100.00%> (ø)
generated/nirfsg/nirfsg/session.py	90.22% <92.85%> (-0.10%)	⬇️

... 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 ee4860e...374e0e3. Read the comment docs.

🚀 New features to boost your workflow:

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