Add modular line search in MadNLP

[frapac]

At the moment MadNLP is using a classical backtracking line-search. We should implement a modular way to specify the line-search algorithm, and give the possibility to the user to use its own line-search algorithm.

[frapac]

Closing this issue as we don't have a particular use-case for a modular line-search. The issue can be reopened if we figure out one.