Add support for dense CUDA solvers #1

1. Add CUDADenseCholesky64Bit, CUDADenseCholesky32Bit, & tests.
   CUDADenseCholesky32Bit uses the legacy versions of potrf/potrs
   in cuSolverDN, while CUDADenseCholesky64Bit uses the new 64-bit
   versions available since Cuda 11.1. The legacy versions are
   provided since some platforms such as the Nvidia Jetsons only
   support Cuda 10.2.
2. Expose CUDA as a new option under DenseLinearAlgebraLibraryType.
   The relevant option to string and string to option helper functions
   are modified accordingly.
3. Add cuda as a dense_linear_algebra_library option in bundle_adjuster
   to demonstrate the use of the new CUDA option.

Change-Id: I23615e1d301df5185ed646b3e33ee802508dae86