[Plugin] SofaCHOLMOD: Add plugin with a supernodal Cholesky direct solver (EigenCholmodSupernodalLLT)

[fredroy]

Add a new optional plugin SofaCHOLMOD providing a direct linear solver, EigenCholmodSupernodalLLT, based on the supernodal sparse Cholesky factorization of CHOLMOD (SuiteSparse), wrapped via Eigen's CholmodSupport.

The dependency on SuiteSparse/CHOLMOD is fully isolated in this plugin so it stays out of the core solvers.

On a ~15k-DOF FEM beam: ~8x speedup over SparseLDLSolver on macOS

Note1: BLAS (important for performance)
Since the factorization delegates to dense BLAS3, performance is dictated by the BLAS backing CHOLMOD, not by SOFA:

• On macOS, SuiteSparse links Apple Accelerate → speedup is always present.
• On Linux, the backend depends on the install BLAS. With the system default one, BLAS the gain mostly disappears; with OpenBLAS/MKL you get the full speedup. See the plugin README for how to check/fix this.
• Multithreaded BLAS tends to oversubscribe on medium systems, so a small thread count is often fastest. A scene-settable numThreads Data is provided to control this at runtime.

Note2: CHOLMOD is double-precision only → enforced at configure time (SOFA_FLOATING_POINT_TYPE=double).

Note3: NOT TESTED ON WINDOWS (YET)

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• it builds with SUCCESS for all platforms on the CI.
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• it does not generate new scene test failures.
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