[config] Allow external qp oases

[olivier-roussel]

This PR enables the use of qpOASES from an external package. A similar approach to the one adopted on SOFA is proposed:

• try to find an external package qpOASES first and use it if found
• if not, fallback to the embedded version in code

Thus, this should keep compatibility for current users.

This is part of some necessary steps to clean the code and to go toward a conda-forge package.

[olivier-roussel]

Missed my rebase onto main, closing this one and will re-open a new PR.