[NXOS] Add Multi-Site fields to ShowNveInterfaceDetail

[behrang-t]

Summary

Enhances the NX-OS ShowNveInterfaceDetail parser to parse three Multi-Site fields that are present in the existing CLI test output but were previously ignored:

• Fabric convergence time
• Fabric convergence time left
• Multisite fabric-advertise-pip l3 configured

The parser now exposes these values as:

• fabric_convergence_time (int)
• fabric_convergence_time_left (int)
• multisite_fabric_advertise_pip_l3 (bool)

Changes

• Added the three fields to ShowNveInterfaceDetailSchema.
• Added regex patterns p27, p28 and p29.
• Added parsing logic for the new fields.
• Updated the existing golden_output4_expected.py fixture to validate the parsed values.
• Added a changelog entry under changelog/undistributed/.

No new raw CLI fixture was required because the existing golden_output4_output.txt already contains all three CLI lines.

Testing

• Confirmed the regression test failed before the parser implementation because the three expected keys were missing.
• Verified golden_output4 passes after the parser change.
• Verified all ShowNveInterfaceDetail folder-based tests pass with a 100% success rate.
• Verified python -m py_compile passes.
• Verified git diff --check passes.
• Ran make json successfully. The generated github_parser.json was not included because local generation introduced unrelated environment-specific path changes.