[mlir][sparse] implement sparse tensor init operation

Next step towards supporting sparse tensors outputs.
Also some minor refactoring of enum constants as well
as replacing tensor arguments with proper buffer arguments
(latter is required for more general sizes arguments for
the sparse_tensor.init operation, as well as more general
spares_tensor.convert operations later)

Reviewed By: wrengr

Differential Revision: https://reviews.llvm.org/D111771