[KNITRO] Add objective bound retrieval for MIP

[eminyouskn]

Fixes # .

Summary/Motivation:

This PR add the objective bound retreival for MIP when KNITRO is used as solver.

Changes proposed in this PR:

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[eminyouskn]

@mrmundt Sorry for adding another PR.

[mrmundt]

@mrmundt Sorry for adding another PR.

RUDE. (Kidding, thanks for doing this 😄 )