Add the OSQP solver in ProxSuite.
Paper: https://web.stanford.edu/~boyd/papers/pdf/osqp.pdf.

Draft PR, until the dense and sparse backends and bindings are added.

TODO:

Dense backend PrimalDualLDLT:

• Tests solution polishing.
• Tests mu update.
• Test: osqp_dense_maros_mezsaros.
• Test: osqp_dense_qp_wrapper.
• Benchmarks.

Dense backend PrimalLDLT:

• Code (inspire from the proxsuite::osqp branch).
• Tests.
• Benchmarks.

Sparse backend:

• Tests.
• Benchmarks.

Bindings with nanobind:

• Code (inspire from the proxsuite::osqp branch).
• Tests.
• Benchmarks.

Refactorize the architecture:

• Refactorize.