feat: Add --definitions flag and artifact

[pdp2121]

High Level Overview of Change

Adds a --definitions CLI flag to xrpld that outputs server definitions as JSON and exits, without starting the server.
CI is updated to export and upload server_definitions.json as a build artifact on every run.

Context of Change

This is particularly valuable for SDK maintainers (e.g. xrpl.js, xrpl-py) who need up-to-date definitions early in a feature's development cycle, before a Docker image is available or a branch is merged to develop.

API Impact

• 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)

Before / After

Before: Generating definitions.json for downstream SDKs required either running a live server_definitions RPC call against a running node, or maintaining a custom regex-based script to extract definitions from source.

After: SDK maintainers can build and use --definitions flag to generate the json file, or simply download the server-definitions artifact directly from any CI run.

Test Plan

CI passes and artifact generated.

[mvadari]

You should refactor this to use the helper function you just added, so there's only one static variable

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.6%. Comparing base (e2e537b) to head (3d5afc9).

Files with missing lines	Patch %	Lines
...pld/rpc/handlers/server_info/ServerDefinitions.cpp	0.0%	3 Missing ⚠️
src/xrpld/app/main/Main.cpp	66.7%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6858     +/-   ##
=========================================
- Coverage     81.6%   81.6%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        75982   75988      +6     
  Branches      7633    7634      +1     
=========================================
+ Hits         61982   61983      +1     
- Misses       14000   14005      +5     

Files with missing lines	Coverage Δ
src/xrpld/app/main/Main.cpp	77.2% <66.7%> (-0.7%)	⬇️
...pld/rpc/handlers/server_info/ServerDefinitions.cpp	96.6% <0.0%> (-1.4%)	⬇️

... and 2 files with indirect coverage changes

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