adds a check to ensure vault asset cap is not exceeded

[Tapanito]

Adds a check to ensure vault asset cap is not exceeded when creating a Loan.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.1%. Comparing base (f7a5f35) to head (bd1cea0).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           ximinez/lending-XLS-66-ongoing   #6124     +/-   ##
================================================================
- Coverage                            79.1%   79.1%   -0.0%     
================================================================
  Files                                 839     839             
  Lines                               71380   71384      +4     
  Branches                             8318    8319      +1     
================================================================
+ Hits                                56476   56478      +2     
- Misses                              14904   14906      +2     

Files with missing lines	Coverage Δ
src/xrpld/app/tx/detail/LoanSet.cpp	91.5% <100.0%> (+0.1%)	⬆️

... and 5 files with indirect coverage changes

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[gregtatcam]

LGTM