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

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

API Impact

• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

[](https://app.codecov.io/gh/XRPLF/rippled/pull/5190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) ```diff @@ 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](https://app.codecov.io/gh/XRPLF/rippled/pull/5190?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) | Coverage Δ | | |---|---|---| | [src/libxrpl/protocol/SField.cpp](https://app.codecov.io/gh/XRPLF/rippled/pull/5190?src=pr&el=tree&filepath=src%2Flibxrpl%2Fprotocol%2FSField.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL2xpYnhycGwvcHJvdG9jb2wvU0ZpZWxkLmNwcA==) | `79.1% <ø> (ø)` | | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/XRPLF/rippled/pull/5190/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) [](https://app.codecov.io/gh/XRPLF/rippled/pull/5190?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF)