[mlir][SCFToGPU] LaunchOp propagate optional attributes
Allow propagating optional user defined attributes during SCF to GPU conversion. Gives opportunity to use user defined attributes in the further lowering. For example setting subgroup size, or other options for GPU dispatch. This does not break backward compatibility and does not require new attributes, just allow passing optional ones.
Differential Revision: https://reviews.llvm.org/D88203