[cmake] Use a better FindMKL.cmake which includes also linker flags.

Also added CMake option OGS_EIGEN_USE_MKL, see https://eigen.tuxfamily.org/dox/TopicUsingIntelMKL.html.

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Some questions:

• Do we need 64-bit indices? See https://www.smcm.iqfr.csic.es/docs/intel/mkl/mkl_userguide/MKL_UG_linking_your_application/Support_for_ILP64_Programming.htm -> Can be enabled with MKL_USE_interface=ilp64 but seems to be not supported by Eigen
• Do we have a benchmark which uses MKL (for testing)? -> ogs-Mechanics/Linear/cube_1e0_mkl

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Fixes #3259 (closed).

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?