Add attribute for multipliers of VectorNonlinearOracle

[blegat]

See jump-dev/MathOptInterface.jl#2887

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.23%. Comparing base (077843b) to head (491da54).

Files with missing lines	Patch %	Lines
ext/IpoptMathOptInterfaceExt/MOI_wrapper.jl	0.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #521      +/-   ##
===========================================
- Coverage   100.00%   99.23%   -0.77%     
===========================================
  Files            5        5              
  Lines         1164     1173       +9     
===========================================
  Hits          1164     1164              
- Misses           0        9       +9     

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[odow]

I think I'm okay adding this as an attribute to MOI directly. It's fairly well defined. Useful across at least three solvers (Ipopt, KNITRO, and Uno). And we would follow our usual dual convention.