Sponsor edge cases

[PeterChen13579]

High Level Overview of Change

NFToken and LoanSet sponsor accounts only sponsor their designated Account, never a counterparty.
Added tests for each case, brokered NFT and LoanSet counterparty scoping, plus that sponsorship honors the free-trust-line allowance and the prefunded reserve budget.

Context of Change

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)

[mvadari]

@PeterChen13579 can you add a description for what issues you're fixing?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (d1a77b4) to head (d10d4f8).

Additional details and impacted files

@@               Coverage Diff               @@
##           xrplf/sponsor   #7558     +/-   ##
===============================================
- Coverage           82.2%   82.2%   -0.0%     
===============================================
  Files               1016    1016             
  Lines              78401   78411     +10     
  Branches            9012    9014      +2     
===============================================
+ Hits               64440   64447      +7     
- Misses             13952   13955      +3     
  Partials               9       9             

Files with missing lines	Coverage Δ
src/libxrpl/ledger/helpers/MPTokenHelpers.cpp	96.5% <100.0%> (+<0.1%)	⬆️
src/libxrpl/ledger/helpers/RippleStateHelpers.cpp	92.9% <100.0%> (ø)
src/libxrpl/tx/transactors/lending/LoanSet.cpp	90.5% <100.0%> (+0.1%)	⬆️
.../libxrpl/tx/transactors/nft/NFTokenAcceptOffer.cpp	93.9% <100.0%> (+<0.1%)	⬆️

... and 2 files with indirect coverage changes

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