fix: include index in server_definitions RPC

[mvadari]

High Level Overview of Change

This PR re-adds the "index" field to the server_definitions RPC, and adds some tests to ensure this is caught in the future.

Context of Change

#5122 removed sfIndex, which had the side effect of removing that field from the server_definitions RPC, and this broke client libraries.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

• Bugfix
• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.9%. Comparing base (9d58f11) to head (8ee6f31).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5190     +/-   ##
=========================================
- Coverage     77.9%   77.9%   -0.0%     
=========================================
  Files          782     782             
  Lines        66616   66620      +4     
  Branches      8161    8137     -24     
=========================================
- Hits         51902   51897      -5     
- Misses       14714   14723      +9     

Files with missing lines	Coverage Δ
src/libxrpl/protocol/SField.cpp	79.1% <ø> (ø)

... and 6 files with indirect coverage changes