Add support for get MOI.NLPBlock

[odow]

@ccoffrin you can replace https://github.com/lanl-ansi/rosetta-opf/blob/1867107cb8b2d036dcd7ddac13637f627dd84ab5/jump.jl#L119C5-L119C55 with

nlp_block = unsafe_backend(model).nlp_data

but once this is merged you'll be able to use

nlp_block = JuMP.MOI.get(unsafe_backend(model), JuMP.MOI.NLPBlock())

[codecov[bot]]

Codecov Report

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

Comparison is base (04178d8) 94.22% compared to head (e0155d0) 94.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #398 +/- ## ========================================== + Coverage 94.22% 94.25% +0.02% ========================================== Files 4 4 Lines 953 957 +4 ========================================== + Hits 898 902 +4 Misses 55 55 ```

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